Chocolate Potato Cake:

1. First off, peel and boil your potato(es). Allow to cool to room temp, then mash. If necessary, push through a sieve to get your potato smooooth. Measure out (or weigh) 1 cup (210g) of mashed tater.
2. Heat oven to 350°F. Grease and line the bottom of two 8" round cake pans with circles of parchment paper; set aside.
3. In the bowl of a stand mixer fitted with paddle attachment, beat butter and sugar on medium-high until pale and fluffy, 5–6 minutes.
4. Add eggs one at a time, mixing until thoroughly combined. Mix in mashed potato and vanilla.
5. In a large bowl, sift together flour, black cocoa powder, cocoa powder, baking soda, salt, and instant coffee.
6. Starting and ending with the flour mixture, add flour mixture and milk to butter mixture alternately. With mixer on low speed, mix until combined, being careful not to overmix. Scrape down bowl as necessary after each addition.
7. Spread batter into prepared pans, smoothing the surface with an offset spatula or the back of a spoon.
   NOTE: It should be roughly 710g of batter per pan.
8. Bake for 28–33 minutes. Cake is done when a wooden pick inserted in the centre comes out clean with only a few fluffy crumbs attached.
9. Allow cake to cool for 20–30 minutes before removing from pan and transferring to a wire rack. Let cake cool completely.

Less Sweet American Buttercream:

1. Once cake is totally cool, let’s make buttercream! In the bowl of a stand mixer fitted with paddle attachment, cream butter and salt on medium speed until fluffy and smooth.
2. Sift in a third of confectioners’ sugar (1 ½-ish cups or 165g), then mix on low until fully incorporated. Scrape down bowl. Repeat two more times with rest of confectioners’ sugar.
3. Mix in Greek yoghurt and vanilla, beating until buttercream is your preferred consistency.

Assembly:

1. Now let’s build the cake! You’ll want the cake layers to be relatively flat so that the mirror glaze can spread evenly, so level your layers, if necessary.
2. Put a small blob of frosting in the middle of your cake board to help keep the cake in place. Then lay down one cake layer.
   NOTE: You MUST use a cake board (ideally 8") for a cake with a mirror glaze. ^[5]
3. Spread a little less than half of frosting across top of cake layer, then carefully place second cake layer on top. Spread most of remaining frosting on top.
4. Use an offset spatula to spread a thin crumb coat over the sides of the cake.
5. Place cake in freezer uncovered for at least 1 hour until it’s chilled to the core and the buttercream is frozen.

Vegetarian Mirror Glaze:

1. Once your cake is nice and frozen, it’s mirror glaze time! Read through these directions multiple times before you begin.
2. Set out 2 large, heatproof bowls. Place white chocolate chips (or chopped white chocolate) in one bowl. Place a large sieve atop the other.
3. In a small bowl, whisk together agar-agar and hot water. Let sit for 5 minutes. (This is called “blooming” the agar.)
4. In a large pot over medium-high heat, combine sugar, corn syrup, and room temp water. Whisking regularly, heat until mixture reaches 150°F, then add bloomed agar-agar.
5. Bring to a boil. Whisking constantly, allow to boil rapidly for 1 minute.
6. Remove from heat and add sweetened condensed milk, whisking until smooth.
7. Pour agar-agar mixture over white chocolate and let sit for a couple minutes so chocolate can begin to soften. Whisk until chocolate is completely melted and mixture is relatively smooth. Blitz with an immersion blender to make glaze even smoother.
8. Pour mixture through sieve into the other large bowl.
9. Decide how many colours of glaze you want, then set out that number of heatproof bowls and spoons. Divide glaze amongst bowls, add gel food colouring to each bowl, then mix each colour with its own spoon (to avoid accidentally mixing colours!).
   NOTE: For the galaxy look, go with blues, black, purples, and/or pinks. We recommend a larger amount of blues and/or black for the background, plus smaller amounts of lighter blues, purples, and/or pinks for cosmos vibes. ^[6]
10. Mirror glaze needs to be around 90–95°F when it’s poured over the cake, so you will need to wait for it to cool down. While the glazes are cooling, stir occasionally to prevent a film from forming. ^[7]
    NOTE: The temperatures of the glaze and cake are both vital to the success of your mirror glaze. The glaze needs to be hot enough to pour, spread, and drip nicely, but not so hot that it melts the buttercream. And the cake must be frozen so it can withstand having 90°F glaze poured over it. Waiting for the correct temperatures is much better than turning your cake into a melty, sticky mess. BE PATIENT, friend.
11. Place a large, rimmed baking sheet on your work surface. Line with parchment or wax paper. This will be catching glaze runoff. In the middle, set a sturdy object that is smaller than 8" in diameter. This will be the cake’s perch, so it needs to be smaller than the cake board to allow the glaze to drip properly. ^[8]
12. Once you begin, you will need to work quickly so the glaze doesn’t set before you’re finished with your design. We recommend going ahead and gathering the following at your work station: large offset spatula, small offset spatula, clean paintbrush, white food colouring in a small bowl, and paper towels.
13. If you want a two-tone space background (e.g., black and navy blue), pour one glaze colour into the other. Gently swirl the glazes together without blending them.
14. When the glazes are at the correct temperature, retrieve cake from freezer and set atop its perch over the lined baking sheet.
15. Starting with your background colour, pour glaze over the centre of your cake, slowly moving the bowl in a circular motion around the cake and towards the edges. Continue until the entire cake (sides included) is fully covered.
16. Using their respective spoons, drizzle other colours over the background, allowing to also drip over the sides of the cake.
17. If you want to blur the colours together a bit, VERY lightly and VERY gently sweep a large offset spatula across the top of the cake. We recommend sweeping in the same direction as your galaxy design.
    NOTE: Again, be very gentle, as it’s easy to mix the colours too much and/or hit buttercream. If this happens, drizzle more glaze on and sweep the spatula over again.
18. To add stars, dip a clean paintbrush into white food colouring. Hold the brush horizontally over the cake, then flick the bristles with your finger to splatter white stars over the cake.
    NOTE: If you’ve never done this before, practice splattering stars on the runoff glaze in the baking sheet to get the technique down so you don’t accidentally add big ol’ blobby stars to your beautiful cake.
19. Let cake sit and drip for 15–20 minutes. Gently remove any excess glaze from the bottom of the cake board with a small offset spatula. Take a moment to admire your handiwork and your reflection in the mirror glaze.
20. Transfer to cake stand or serving platter. Enjoy immediately or store in fridge for up to one day.
    NOTE: The gorgeous shine will become matte after 24 hours, so keep your serving day/time in mind while deciding when to glaze your cake.