Beat the egg whites and granulated sugar together in a medium bowl on high speed for 5 minutes or until stiff peaks form. Set aside. In another bowl, beat the egg yolks, brown sugar, and peppermint extract together until pale and creamy, about 2 minutes.

Sift the flour, cocoa powder, baking powder, and salt together into a large bowl. Pour the melted butter, coffee, and egg yolk mixture over the dry ingredients. Beat everything together on medium speed until completely combined. Using a rubber spatula or wooden spoon, gently fold in the egg whites until completely combined. Avoid over-mixing and deflating those whites. Batter will be very light.

Spread batter evenly into prepared pan. It will be a very thin layer. Shimmy the pan on the counter to smooth out the top. Bake for 10 minutes or until the top of the cake gently springs back when touched with your finger. Do NOT over-bake.

As the cake bakes, place a thin kitchen towel flat on the counter. Sprinkle with a light coating of powdered sugar. Once the cake comes out of the oven, immediately invert it onto the towel. Peel off the parchment paper that was on the bottom of the cake as it baked. Starting with the narrow end, slowly and gently roll the cake up with the towel. The cake will be warm. Allow the cake to cool completely rolled up in the towel. Place in the refrigerator to speed it up, about 2-3 hours.

With a handheld or stand mixer fitted with a paddle or whisk attachment, beat the butter on medium speed until creamy – about 2 minutes. Add powdered sugar, cocoa powder, heavy cream, salt, and peppermint extract. Beat on low speed for 30 seconds, then increase to high speed and beat for 1 full minute. Add 1/4 cup more confectioners’ sugar or cocoa powder if frosting is too thin or another Tablespoon of cream if frosting is too thick.

Remove the cake from the refrigerator. Dust the top with powdered sugar, slice into half-inch pieces, and enjoy!