Preheat oven to 350°F. Line an 8x8-inch baking pan with parchment paper. Microwave butter in a microwave safe bowl until melted and hot.
Place dark chocolate chips, ¼ cup mint chocolate chips, 3 tablespoons cocoa powder, and espresso powder in a bowl. Pour hot melted butter and stir until chocolate is melted and mixture is smooth.
In a large bowl, beat the sugars, eggs, vanilla, and salt for about 10 minutes.
Slowly pour the melted chocolate mixture into the whipped egg mixture while beating gently. Mix just until fully incorporated.
Sift in the flour and cocoa powder. Gently fold them into the batter using a spatula. Mix just until combined. Do not overmix.
Fold in ¾ cup mint chocolate chips, reserving a small handful for topping later.
Pour the batter into the prepared pan and smooth the top. Bake for 25 minutes.
Remove brownies from the oven and slam the pan on the counter once or twice. This helps create a denser, fudgier brownie with a more pronounced crinkly top.
Sprinkle reserved mint chocolate chips on top. Return the pan to the oven and bake for about 20 more minutes. The edges should be set while the center still looks slightly underbaked.
Let the brownies cool completely in the pan before cutting. Slice, serve and enjoy!