Preheat the oven to 350°F (175°C) and prepare a baking dish.
In a large bowl, mix melted butter, granulated sugar, and brown sugar until combined.
Add the eggs and vanilla to the butter and sugar mixture, and blend until smooth.
In a separate bowl, whisk together flour, cocoa powder, baking soda, and salt.
Gradually add the dry mixture to the wet ingredients, mixing until just combined.
Fold in the chocolate chips until evenly distributed throughout the dough.
Pour the cookie dough into the prepared baking dish, spreading evenly across the bottom.
In another bowl, combine cocoa powder, melted butter, and sugar for the brownie layer.
Add eggs, vanilla, and peppermint extract to the brownie mixture, whisking until smooth.
Mix in the flour and baking soda until just combined, then fold in mint chocolate chips.
Pour the brownie batter over the cookie layer, spreading it to cover completely.
Bake for 25-30 minutes, or until a toothpick inserted comes out moist but not wet.
Let them cool in the pan for at least 15 minutes before slicing into squares.
