Preheat oven to 300 degrees (150 degrees C). Grease one 9x13 inch baking pan.
In a large bowl, cream together the butter, brown sugar, white sugar, vanilla, and eggs. Sift together the flour, baking powder, and salt; stir into the egg mixture until well blended. Spread mixture evenly into the prepared baking pan.
Bake for 45 minutes in the preheated oven. A toothpick should come out clean when inserted. Cut into squares and roll in confectioners' sugar while they are still hot.