Grease a heart-shaped cakes pan, or line and grease a large casserole pan - about 9x13". Set aside.
In a small bowl, sift together the cocoa powder, salt and baking powder. Set aside.
Place the butter in a small saucepan over medium heat and cook for 8-10 minutes until slightly golden brown. Small pieces of milk fat will turn a chocolate brown and that's okay.
Remove the pan from the stove and immediately stir in the white sugar with a wooden spoon.
Add in the cocoa mixture and then allow to cool to room temperature, about 5-8 minutes.
Meanwhile, prepare the cheesecake mixture.
Combine the egg, cream cheese, sugar and mint extract and beat with an electric mixer until smooth.
Color to preference with the optional green food dye and set aside.
Returning to the brownie batter, add in the eggs one at a time, then the vanilla and flour, beating thoroughly after each addition. Beat for 2 minutes before adding in the chocolate chips.
Fold the brownie mixture into the prepared baking pan.
Add dollops of the cheesecake mixture overtop of the brownie mixture and use a spoon to fold the two to create a marbled design.
Cover with tinfoil and bake for 22-25 minutes, until the cheesecake seems set.
Check the doneness of the brownies with a toothpick - you may need to bake uncovered for an additional 5 minutes.
Cool before drizzling with melted chocolate, optional.