Preheat your oven to 350°F (175°C). Grease and flour two 9-inch round cake pans or line them with parchment paper. In a large bowl, whisk together the flour, cocoa powder, baking powder, baking soda, and salt.

In a separate bowl, beat the granulated sugar, vegetable oil, butter, and vanilla extract until well blended. Add eggs one at a time, mixing well after each addition. Mix in the milk and buttermilk.

Slowly add the dry ingredients to the wet ingredients, stirring until just combined. Stir in the boiling water (carefully), then mix in the peppermint extract. Batter will be thin this is normal. Divide the batter evenly between the prepared pans.

Bake for 26–30 minutes, or until a toothpick inserted into the center comes out clean. Let cakes cool in pans for 10 minutes, then remove and cool completely on wire racks. In a small saucepan over medium heat, whisk together 2½ tbsp flour and ½ cup milk until thickened. Remove from heat and let cool completely.

In a mixing bowl, beat softened butter with powdered sugar until light and fluffy. Add in the cooled flour-milk mixture and beat until smooth. Stir in vanilla extract, peppermint extract, and green food coloring (if using).

In a separate bowl, whip heavy cream to stiff peaks, then gently fold into the buttercream mixture to make it airy. Place one cake layer on a serving plate. Spread half the frosting mixture evenly over the top.

Place the second cake layer on top, and frost the top and sides with the remaining frosting. Garnish with mini chocolate bon bons around the edges or on top.