Preheat oven to 350 degrees F. Spray an 8-inch square pan with vegetable oil cooking spray.
In a small saucepan, melt the butter. Turn off the heat, add the brown sugar and stir until smooth. Stir in the egg. Stir together the flour and baking powder and stir into the brown sugar mixture. Stir in the vanilla and pecans. Pour the batter into the prepared pan and bake for 20 minutes. When cool, dust the top with a sifting of confectioners' sugar.