HEAT oven to 350ºF.
MELT 1/2 cup butter. Mix 1-1/4 cups flour, dry gelatin mix and brown sugar in medium bowl until blended. Stir in melted butter; set aside. Mix remaining flour, baking powder and baking soda until blended.
BEAT remaining butter and granulated sugar in large bowl with mixer until light and fluffy. Add eggs, 1 at a time, beating well after each. Blend in sour cream and lemon zest. Gradually beat in baking powder mixture.
SPOON into 9-inch round pan sprayed with cooking spray; top with gelatin mixture.
BAKE 40 to 45 min. or until toothpick inserted in center comes out clean. Cool completely.