Make the mint filling: In a large bowl, whip cream cheese until smooth. Add sugar and egg white until combined. Add extract and green food coloring. Set aside while you make the brownies.
Make the brownies: Preheat the oven to 350 degrees, and line a 9 x 9 pan with parchment paper.
In a large bowl over a double-boil, melt butter and chocolate. Add sugars and remove from heat. Stir in eggs and vanilla. Add flour, salt and baking powder to combine. Pour batter into prepared pan. Drop nine 2-inch circles of batter evenly across the pan and swirl with a knife or chopstick to create a marbled effect. Place Thin Mint pieces evenly across the brownies. Bake for 30 to 35 minutes until set and slightly shiny. Brownies will keep up to 5 days covered.