1. Combine first 5 ingredients. Set aside.
2. Beat butter at medium speed with an electric mixer until creamy. Gradually add sugars, beating until well blended. Add egg, beating until blended. Reduce speed to low; gradually add flour mixture, beating just until blended. Add molasses and milk, beating just until blended. Spread batter in a lightly greased 13- x 9-inch pan.
3. Bake at 350° for 25 minutes or until a wooden pick inserted in center comes out clean. Cool in pan on a wire rack. Spread with Lemon-Cream Cheese Frosting. Cut into squares. Garnish with grated lemon rind, if desired. Cut each bar in half, forming 2 triangles if desired.
Note: Frosted bars may be stored in an airtight container in the refrigerator 5 to 7 days. If you decide to freeze these bars, do not frost them before freezing. Add the frosting to partially thawed bars, and then cut into shapes.