Heat oven to 350°F. Line 2 baking sheets with parchment paper; set aside.
In a large heatproof bowl, whisk together, flour, baking soda, and salt. Set aside.
In a medium pot over medium heat, melt peanut butter and butter whilst stirring frequently. Once totally smooth, whisk in brown sugar.
Remove from heat, then whisk in milk, followed by egg and vanilla.
In the centre of the dry ingredients, make a well, then pour in peanut butter mixture. Mix just until combined, making sure to hunt down any hidden pockets of flour.
Gently plop ¼-cup mounds of batter 2" apart on prepared baking sheets. (This 2-ounce disher was perfect for portioning.)
Bake for 12–14 minutes or until cookie-cakes are puffed up and spring back when lightly pressed.
Allow cookie-cakes to cool for 5 minutes on baking sheets, then transfer to wire rack to cool completely.
Once your cookie-cakes are totally cool, it’s frosting time! In the bowl of a stand mixer fitted with paddle attachment, cream butter and salt on medium speed until fluffy and smooth.
Sift in a third of confectioners’ sugar, then mix on low until fully incorporated. Scrape down bowl. Repeat two more times with rest of confectioners’ sugar.
Mix in Greek yoghurt and vanilla, followed by Marshmallow Fluff. Mix or fold in Concord grape spread. NOTE: If your kitchen is hot and your frosting seems soft, place it in the fridge for a bit. Depending on how long it’s in the fridge (e.g., a few hours), you may need to re-beat the frosting to get it smooth before using.
Spread or pipe frosting on half of the cookie-cakes. (We used an Ateco 804 tip.) If desired, slather additional Concord grape spread on other half of cookie-cakes. Taking one frosted cookie-cake and one jammed cookie-cake, gently press each whoopie pie together.
Notes
Peanut butter cookie-cakes source: Katherine Keenan for Yankee Magazine
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