One of my favorite things about cooler temperatures is all things mint (or peppermint). We add that glorious flavoring to hot cocoa, to brownies, to cookies, and most definitely to cakes!
Preheat the oven to 350°F. Prepare two 8-inch round cake pans. (Line with parchment paper, then butter and flour the pans, or spray with nonstick baking spray.)
Sift the flour, sugar, cocoa, baking soda, baking powder and salt into the bowl of a stand mixer fitted with a paddle attachment and mix on low speed until combined.
In another bowl, whisk together the buttermilk, oil, eggs, and vanilla. With the mixer on low speed, slowly add the wet ingredients to the dry. With mixer still on low, add the coffee and stir just to combine, scraping the bottom of the bowl with a rubber spatula.
When cakes are completely cool, level each layer of chocolate cake and then cut each layer in half. You should now have four layers of cake. Chill until ready to assemble.
Chop the chocolate and place it in a heatproof bowl set over a pan of simmering water. Stir until just melted and set aside until cooled to room temperature.
In the bowl of a stand mixer fitted with a paddle attachment, beat the butter and vanilla on medium-high speed until light yellow and fluffy, about 3 minutes.
Turn the mixer to low, gradually add the confectioners' sugar, then beat at medium speed, scraping down the bowl as necessary, until smooth and creamy. Dissolve the coffee powder in 2 teaspoons of the hottest tap water. On low speed, add the chocolate and coffee to the butter mixture and mix until blended.
Blend peppermint extract, 2 tablespoons milk, and salt. Gradually stir remaining milk into frosting until desired spreading consistency is reached.
Melt the candy melts and the shortening in a large microwave-safe bowl for 1-minute increments, then in 15-second increments, stirring after each heating.
Pour half of the melted mixture over the cookie sheet and spread out with an offset spatula. This is important, as you want the layer to be as even and level as possible. Thin is good, but not translucent.
Place it into the fridge to set for 1 to 2 minutes. You can also leave it on the counter for about 10 minutes. The mixture needs to be at the perfect temperature for curls to form. If it’s too cold, they will just break. If it’s too warm, they will just be blobs. Press your finger into the mixture. If it leaves a mark, the mixture is too soft.
Using a flat head screwdriver (clean!), press the screwdriver into the mixture at a 45-degree angle and then move forward slowly. Curls should form. Immediately place curls on a cooled plate or in the refrigerator so that they hold their shape. Chill until ready to assemble cake.
Place remaining chocolate buttercream into a piping bag fitted with a large, open round tip (such as a 2A tip).
Pipe out dollops of frosting around the top of the cake. Sprinkle mint curls over top. Chill cake until ready to serve.