Preheat oven to 350 F. Line an 8 inch metal pan with foil and mist the bottom with cooking spray.
Melt the butter in a microwave-safe bowl. Whisk in the sugar, then whisk in the eggs and vanilla. In a separate bowl, mix together the flour, baking powder, salt and cocoa powder. Gradually add flour mixture to the batter. Stir in the pecans (if using). Pour into pan and bake for 20-25 minutes (I baked for 20). Pull from oven and make frosting.
Frosting: Stir together cocoa powder and powdered sugar. If you have time, you might want to sift it. Melt the butter in a saucepan. Add the milk. Stir it into the chocolate/sugar mixture. Stir in the vanilla. Add pecans if using. Pour over the warm brownies and kind of spread it a little to cover. Garnish with pecan halves.
Let brownies cool completely, then chill for several hours (this makes them easier to cut and neater). Lift from pan, set on a cutting board, trim edges and cut into squares.