Preheat oven to 350°.
Coat a 9-inch round cake pan with cooking spray, and line bottom with wax paper. Coat wax paper with cooking spray; set aside.
Beat egg at medium speed of a mixer until foamy; gradually add sugar, beating well. Combine buttermilk, margarine, and vanilla; set aside. Combine flour, baking powder, baking soda, and salt. With mixer running at low speed, add flour mixture to egg mixture alternately with buttermilk mixture, beginning and ending with flour mixture.
Pour batter into prepared pan. Bake at 350° for 28 minutes or until a wooden pick inserted in center comes out clean. Let cool in pan 10 minutes on a wire rack; remove from pan. Let cool completely on wire rack.
Combine yogurt and brown sugar; stir well. Using a serrated knife, split cake in half horizontally; place bottom layer, cut side up, on a serving plate. Spread with half of yogurt mixture; top with strawberries. Combine banana and lemon juice; toss gently. Drain banana; discard juice. Arrange banana over strawberries; drizzle with remaining yogurt mixture. Top with remaining layer. Sift powdered sugar over top of cake.