preheat oven to 350 degrees Fahrenheit. Spray a 13 by 9 inch baking pan with nonstick baking spray with flour.
in a large bowl, beat butter, sugar, and lemon zest with a mixer at medium speed until fluffy. Add eggs one at a time, beating well after each addition. Beat in lemonade concentrate until combined
in a medium Bowl whisk together flour and baking soda. Gradually add flour mixture to butter mixture alternately with milk beginning and ending with flour mixture stir in sour cream spoon batter into prepared pan
bake for 30 minutes or until a wooden tooth pick inserted in the center comes out clean. Let cool completely. Spread reliving Aid frosting. Cut into squares, and garnish with lemon just before serving, if desired.