Preheat oven to 350° F. Prepare a 9x13 baking pan with cooking spray and parchment paper. The parchment paper should go up the sides of the pan so the brownies are easily lifted out of the pan.
In a microwave-safe bowl, combine unsalted butter and bittersweet/semisweet chocolate chips. Microwave in 30 second intervals, mixing in between, until the chocolate and butter are melted and fully combined.
In a large mixing bowl, combine granulated sugar, brown sugar, eggs, vanilla extract, salt, and espresso powder. Using an electric hand mixer, mix on low speed until the ingredients combine and then turn the mixer to high speed and whip for 2 to 3 minutes until the mixture is light and fluffy.
Slowly drizzle in the chocolate mixture, mixing on low speed until the chocolate is fully incorporated with the egg mixture.
Add in the all-purpose flour and dark cocoa powder. Mix on low speed until the dry ingredients are fully incorporated with the wet ingredients. Do not overmix. Use a spatula to scrape the sides of the bowl to ensure that all the ingredients are well combined.
Pour the brownie batter into your prepared 9x13 baking pan and spread evenly.
Bake for approximately 25-30 minutes or until a toothpick inserted in the center of the brownies comes out with wet crumbs and not batter. Cool the brownies in the pan completely before adding on the mint buttercream layer.