In a medium bowl, beat the softened cream cheese with a handheld or stand mixer until smooth.
Add sugar, egg yolk, peppermint extract, and food coloring (if using). Beat until combined and creamy.
Fold in mini chocolate chips.
Spread the mint cheesecake mixture evenly over the brownie batter in the pan.
Drop small spoonfuls of the reserved brownie batter on top and swirl gently with a knife.
Bake for 35–40 minutes or until a toothpick inserted in the center comes out mostly clean.
Cool completely in the pan before cutting into squares. Refrigerate leftovers in an airtight container for up to 5 days.