Preheat oven to 350° F. Butter the bottom and sides of a 9″ x 13″ pan. Line bottom of pan with parchment paper, and then butter the parchment paper. Add a bit of flour to coat the entire inside surface of the pan, tap out the excess, and set pan aside. This will allow you to turn the cake out onto a platter. If you want to leave the cake in the pan, skip the parchment paper and follow the rest of the steps.

In the bowl of a stand mixer fitted with a paddle attachment, add flour, sugar, cocoa powder, baking powder, baking soda, espresso powder, and salt. Mix on low speed to combine dry ingredients well.

Add buttermilk, vegetable oil, eggs, and vanilla to dry mixture and mix together on medium speed until well combined. Reduce speed to low and carefully add hot coffee to the cake batter. Once the coffee is incorporated, scrape the bottom of the bowl with a rubber spatula and then turn mixer speed to high and beat for 1 minute.

Pour cake batter into the prepared pan. Bake for about 40 minutes, until the center is mounded and firmed up, and a toothpick inserted in the center comes out clean. Remove cake from oven and let cool for about 15 minutes. Place serving platter upside down on top of cake pan, and invert cake onto it. Let cake cool completely before spreading the top with the fluffy mint chocolate chip buttercream.

In the bowl of a stand mixer fitted with a paddle attachment, whip butter on medium-high speed until very fluffy, about 8 minutes, occasionally scraping down the sides and bottom of the bowl.

Add powdered sugar and salt, and beat on low to incorporate. Add half and half, mint extract, and vanilla extract, mixing on low until combined. Increase mixer speed to medium-high and beat until very light and fluffy, about 5 to 6 minutes, occasionally scraping down the sides and bottom of the bowl.

Add green coloring, if desired, and mix until incorporated. With a rubber spatula, fold in the chopped chocolate just before spreading over the chocolate cake.