Preheat oven to 300°F. Prepare three 8-inch cake pans with parchment paper and grease the sides.

Add flour, sugar, cocoa powder, baking soda, and salt to a large bowl and whisk. Add eggs, milk, vegetable oil, and vanilla. Mix well.

Pour boiling water into batter slowly, mixing continuously until smooth. Divide batter evenly into prepared pans.

Bake for 30-35 minutes until a toothpick inserted in center comes out clean. Cool cakes completely on wire racks.

For the buttercream, beat butter until smooth. Gradually add powdered sugar and mix well. Add heavy cream, vanilla, mint extract, and salt. Mix until fluffy.

Color the buttercream with gel icing colors, then fold in chopped chocolate until evenly incorporated.

Assemble the cake layers, spreading buttercream between each. Frost the outside of the cake with remaining buttercream.

Make the ganache by heating the heavy cream until boiling, then pouring over chopped chocolates. Stir until smooth. Drizzle over cake as desired.