Cream margarine; gradually add the sugar, beating well with an electric mixer set at medium speed. Add egg whites, 2 at a time, beating well after each addition. Combine yogurt, brandy, and vanilla; stir well. Add to creamed mixture, beating well. Combine flour, soda, and salt; gradually add to yogurt mixture, beating an additional minute. Pour batter into a 12-cup bundt pan coated with cooking spray. Bake at 325 degrees F for 55 minutes or until a wooden pick inserted in center comes out clean. Cool in pan 15 minutes; remove from pan, and cool on a wire rack. Sprinkle powdered sugar over cake.