For the cake: Preheat oven to 350 degrees. Butter 2 9-inch round baking pans and line with a round of parchment paper, butter parchment. Lightly dust pans with cocoa powder shaking out excess. Set aside.
In a large mixing bowl whisk together flour, sugar, cocoa powder, baking soda, baking powder and salt.
Add buttermilk, vegetable oil, eggs, peppermint extract and vanilla extract, then using an electric hand mixer blend mixture until on low until combined then increase speed to medium and mix 2 minutes. Mix in boiling water.
Divide batter evenly among prepared baking pans. Bake in preheated oven 30 - 35 minutes then transfer to wire racks to cool for 10 minutes.
Run knife around edges of cake to ensure the are loosened then invert onto wire racks to cool completely. Once cool frost with Peppermint Buttercream Frosting. Store cake in an airtight container at room temperature or in refrigerator.
For the frosting: In the bowl of an electric stand mixer whip butter until pale and fluffy. Add in 3 Tbsp of the cream, the peppermint extract and vanilla extract and 1 cup of the powdered sugar and mix until combined.
Add remaining powdered sugar and mix on low speed until blended and increase to medium-high speed and whip until light and fluffy, while adding in additional cream 1/2 Tbsp at a time to reach the right consistency.