Prep
Preheat oven to 350°F (175°C).
Line a 9x9-inch baking pan with parchment paper.
Lightly grease the parchment.
Make the Brownie Batter
In a bowl, whisk melted butter and sugar until combined.
Mix in cocoa powder until smooth.
Add eggs one at a time, stirring well after each addition.
Stir in vanilla and salt.
Fold in flour until just combined. Do not overmix.
Make the Mint Cream Cheese Layer
In a separate bowl, beat softened cream cheese and sugar until smooth.
Add egg and peppermint extract.
Mix until creamy.
Add green food coloring if desired.
Assemble
Pour about ¾ of the brownie batter into prepared pan.
Spread mint cream cheese mixture evenly over it.
Drop spoonfuls of remaining brownie batter on top.
Use a knife to gently swirl layers together.
Bake
Bake for 30–35 minutes.
Edges should be set and center slightly soft.
A toothpick should come out with a few moist crumbs (not wet batter).
Cool
Cool completely before slicing.
For clean cuts, refrigerate for 1 hour before serving.