Heat oven to 350°F. Grease 9-inch square baking pan. Prepare brownies according to package directions, using egg, oil, and water. Spread batter evenly in pan. Bake 25 to 35 minutes, or until a toothpick inserted in center comes out clean. Cool 10 minutes in pan. Invert onto wire rack and cool completely.
Cut brownies into 9 squares. Sprinkle with layer of powdered sugar. Using cat and bat stencils, sprinkle with lavender and orange sugar.