Preheat the oven to 350°F. Grease and flour two 9-inch round cake pans. In a large mixing bowl, combine the flour, cocoa powder, sugar, baking powder, baking soda, and salt. In another bowl, cream the butter and sugar until fluffy. Add the eggs, one at a time, then stir in the vanilla and mint extracts. Gradually add the dry ingredients, alternating with milk, until well blended.

Divide the batter evenly between the prepared pans. Bake in the preheated oven for 30-35 minutes, or until a toothpick inserted into the center comes out clean. Allow the cakes to cool in the pans for 10 minutes before transferring to a wire rack to cool completely.

In a large bowl, beat the butter until creamy. Gradually add the powdered sugar, alternating with heavy cream, until the frosting is smooth and spreadable. Add the mint extract and green food coloring, if desired.

Once the cakes are cooled, place one layer on a serving plate and spread a generous amount of mint frosting over the top. Place the second layer on top and frost the entire cake with the remaining frosting.

Garnish with chocolate shavings or mini chocolate chips if desired. Slice and serve with a cold glass of milk or a cup of coffee.

Store any leftover cake in an airtight container at room temperature for up to 3 days, or refrigerate for up to a week.