Preheat oven to 350 degrees F. Grease an 8"x8" baking pan and set aside. In a saucepan or microwaveable bowl, melt butter. Mix in brown sugar, vanilla, and milk. Once combined, add egg and mix well. Add flour, baking powder, and salt and mix until combined. Pour into prepared pan. Bake at 350 degrees F for 22-25 minutes. Cool in pan on a wire rack.