Preheat the oven to 325°F and grease an 8X8 baking dish.
In a large bowl, add the room temperature butter and brown sugar, and cream them together. Add the egg and mix it in.
Add ¾ cup powdered sugar, 2 tsp vanilla, 2 tsp baking powder, ¼ tsp salt and mix together.
Add in 2½ cups flour slowly and mix until completely incorporated.
Divide the dough in half. Press one half into the bottom of your prepared pan. On your counter, roughly roll or press the other half into an 8"x8" square. It doesn't have to be perfect, but it will be much easier to form the second layer to fit outside the pan.