In a medium bowl, whisk together flour, baking powder, ground ginger, baking soda, and salt. Set aside.
In a liquid measuring cup, mix together molasses and buttermilk. Set aside.
In the bowl of a stand mixer fitted with paddle attachment, cream butter and sugar on medium-high until light and fluffy.
Add egg, mixing until thoroughly combined. Mix in vanilla.
Starting and ending with the flour mixture, add dry ingredients and wet ingredients 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.
Lightly brush melted butter onto cast iron skillet.
Drop a scant ⅓ cup of batter into skillet. (This will be the first of 7 layers.) Cover batter bowl with a damp dish towel to keep batter moist. Using an offset spatula, spread batter into a thin, even circle that is 6-ish inches in diameter.
Bake 10–12 minutes or until layer springs back when lightly touched. (The layers only rise a teeny-tiny bit.)
Once cake layer is cool enough to handle, remove from skillet and place onto serving plate. Brush lightly with melted butter. Using an offset spatula or spoon, spread 2 Tbsp of apple butter over the cake layer—from edge to edge. NOTE: Unlike a standard layer cake, you don’t need to wait for these cake layers to cool before assembling the cake.
With a dry towel, remove any stray crumbs from cast iron skillet. Lightly brush skillet with melted butter, then repeat process to bake and assemble remaining 6 cake layers. NOTE: The skillet does not need to cool down between baking layers.
After you place the seventh and final cake layer on top, brush with butter and spread remaining apple butter on top.
Store finished cake in an airtight container, allowing to rest at room temperature for 1–2 days before cutting.
If you don’t have buttermilk, mix 3 Tbsp + 2 tsp (52g) whole milk with 1 tsp white vinegar and let sit for 15 minutes.
Using a cast iron skillet is the traditional method, which is why we went this route. It does take a while because you’re only baking one layer at a time. If you would like to bake multiple layers at once, you can use greased cake pans or parchment-lined baking sheets.
It’s much easier to form the batter into a circle if you use a very light touch. Too much pressure causes the circle to break and slide around in the buttered skillet. (Don’t worry; you’ll quickly get the hang of it.)