Preheat oven to 350°.
Coat an 8-inch square baking pan with cooking spray; dust bottom of pan with 2 teaspoons flour.
Place butter in a large microwave-safe bowl. Cover and microwave at HIGH 1 minute or until butter melts. Add sugar, stirring with a whisk. Add molasses and eggs; stir with a whisk until smooth. Add ginger, pumpkin pie spice, baking soda, mustard, and salt; stir well with a whisk. Add milk and orange rind; stir well with a whisk. Lightly spoon 1 1/2 cups flour into dry measuring cups; level with a knife. Add flour, stirring just until blended (do not overstir). Pour batter into prepared pan.
Bake at 350° for 30 minutes or until a wooden pick inserted in center comes out clean. Cool in pan 10 minutes on a wire rack. Cut into squares. Serve with whipped topping.