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.
Notes
Here's another method for making the frosting. I sometimes use the one above, and I sometimes use this one depending on my mood. I think this one's a little smoother.
-----------Sift the confectioners’ sugar into a mixing bowl and set aside. You can skip the sifting step if you must, but sifting helps it to blend easier and the icing will be smoother. Melt the butter in a saucepan set over medium heat. When butter is almost melted, add the cocoa and milk and bring just to a boil. As soon as the mixture reaches a good boil, remove from heat. Pour the hot boiling cocoa mixture into the bowl of sifted confectioners’ sugar. Add the vanilla and stir with a wooden spoon until smooth. Work quickly so that icing won’t start to set in the bowl. Pour over the brownies and let sit until firm.