Recipe

To make the wafers in a food processor:

1. Combine the flour, cocoa powders, sugar, salt, and baking powder in the work bowl of a food processor, pulsing until mixed. Add the butter and run the machine until the mixture is powdery. Add the egg and vanilla and run the machine until the dough begins to clump and ball together.

To make the wafers with an electric mixer:

1. Beat the butter and sugar together until combined. Add the egg and vanilla and beat until smooth. Add the baking powder, salt, and cocoa powders and beat until combined. Add the flour and mix just until it disappears.

Both methods:

1. Heat the oven to 350°F. Divide the dough into 6 equal pieces. Roll the first between 2 pieces of parchment paper until very, very thin and just over 7 inches across. Slide onto a board (parchment paper and all) and place in the freezer for 10 minutes, until firm.
2. Once firm, peel back the top piece of parchment paper (it should now come off cleanly when pulled gently) and use a stencil or bowl with a 7-inch rim to trim it into a neater circle. Slide the cookie round and the lower piece of parchment paper onto a baking sheet. Bake for 10 minutes and let it cool completely on paper, which you can slide onto a cooling rack so that you can use the tray again. Repeat with the remaining 5 pieces of dough.
3. It sounds like a lot of work, but the best thing is to get into a pattern where one piece is being rolled while another is freezing while the third one bakes and a fourth is cooling, so you’re never working with more than one piece at a time. By the time one piece bakes, the next is ready to leave the freezer.

While the cookies cool, make the peanut butter whipped cream:

1. In a large bowl, beat the peanut butter, vanilla, salt, and sugar until smooth. Beating the whole time, slowly add the heavy cream, one small splash at a time, until the peanut butter cream mixture is loose enough that you can add the rest of the cream without breaking it into clumps. Whip the cream, watching it carefully as it’s very easy to over beat with an electric mixer, until soft peaks form.

Assemble the cake:  

1. Place the first cookie on a cake stand. If it’s sliding around, as cookies do, put a dab of whipped cream down first. Once it softens the cookie, it will make it stick. Thickly frost the first cookie all the way to the edges with about 1/2 cup peanut butter cream.
2. Repeat with the remaining cookies, decoratively swirling the top cookie. Garnish with sprinkles or candy.
3. Place the cake in the fridge overnight or, ideally, for closer to 24 hours so that the cookies soften into cake layers. A knife dipped in warm water will make clean cuts.