With an electric mixer cream together the butter and sugar for 3 minutes, until light and fluffy. Add the egg, milk and vanilla extract, beat for another minute. Stir together the flour, baking powder and salt, then add that to the butter mixture. Mix on low speed just until everything is combined. Spread half of the batter onto the bottom of the prepared pan. The batter will be very thick and sticky, so it helps if you grease your hands a bit and then press the batter down. You will get a very thin layer but it will rise up during baking. Set aside and make the fillings.