Step #2 Into medium mixing bowl sift together flour & baking powder; stir in the lemon peel & set aside.
Step #3 In large mixing bowl mix sugar, eggs, & margarine and, using electric mixer at medium speed, butter, beat until mixd; increase peed to high & beat until mixture is light & fluffy & double in volume.
Step #4 Add flour mixture alternately with milk, beating at lowish speed until batter is smooth.
Step #5 Transfer batter to prepared pan & bake for 50 to 55 mins (until browned & a cake tester inserted in center, comes out clean).
Step #6 Let cake cool in pan for 5 mins; invert cake onto a wire rack & let cool completly.
Step #7 To serve.
Step #8 garnish with lemon slices & cut into 12 equal slices.