Preheat oven to 350°. Cream butter and brown sugar until light and fluffy, 3-4 minutes. Beat in the vanilla. Beat in the eggs, 1 at a time, beating well after each addition. In another bowl, whisk together flour, baking powder, spices and salt; gradually beat into creamed mixture. Spread into a greased 9-in. square baking pan.
Mix topping ingredients; sprinkle over top. Bake until set and golden brown, 35-40 minutes. Cool completely in pan on a wire rack. Cut into bars.