Line an 8x8 square baking pan with parchment paper and set aside.
In a large bowl, cream together the butter and brown sugar.
Add in the egg, egg yolk, and vanilla extract. Mix until well combined.
Stir in the flour and baking powder until well combined.
Fold in the chocolate chips.
Pour the batter into the baking dish and bake for 25-30 minutes.
Let cool and then cut into 9 or 16 squares before serving.