With the rack in the middle position, preheat the oven to 180 °C (350 °F). Line the bottom of a 20-cm (8-inch) square baking pan with a strip of parchment paper, letting paper hang over two sides. Butter the other two sides.
Pour half of brownie batter in the pan. Layer with mint chocolate squares. Top with remaining batter.
Bake for about 35 minutes or until a toothpick inserted in the centre of the cake comes out with a few lumps and not completely clean.
Let cool in the pan. Unmould from pan and cut into squares.
A quick and easy way to give your brownies a different flavour.