How to Make It
Preheat oven to 350°.
Coat an 8-inch square baking pan with cooking spray; line bottom of pan with wax paper. Coat wax paper with cooking spray; set aside.
Lightly spoon flour into dry measuring cups; level with a knife. Combine flour, baking powder, and salt.
Beat butter with a mixer at medium speed until creamy; gradually add granulated sugar, beating until fluffy. Add egg and egg white, and beat at medium speed just until blended. Add flour mixture to sugar mixture alternately with 3/4 cup milk, beginning and ending with flour mixture. Stir in orange rind and vanilla.
Pour batter into prepared pan. Sharply tap pan once on counter to remove air bubbles. Bake at 350° for 29 minutes or until a wooden pick inserted in center comes out clean; cool in pan for 10 minutes on a wire rack. Remove from pan, and carefully peel off wax paper; cool completely on wire rack.
Combine powdered sugar and next 3 ingredients in a large bowl. Beat with a mixer at low speed just until blended, and stir in orange marmalade.
Place cake on a plate; spread frosting over top and sides of cake.
Oxmoor House Healthy Eating Collection