Preheat oven to 350 degrees. Grease or line with foil a 9 x 9 baking dish.
In a large bowl or the bowl of a stand mixer, cream together butter and sugar.
Add vanilla and eggs, and mix until smooth.
Add flour, cocoa, baking powder, and salt, and mix until just combined.
Gently fold in creme de menthe pieces, reserving a few to sprinkle on top.
Spread evenly in prepared pan, top with remaining creme de menthe pieces, and bake 20-25 minutes, or until a toothpick inserted about two inches from the side of the pan comes out clean.
Let cool and then cut into squares (I usually cut 16 squares, since they are pretty rich).