HEAT oven to 350°F. Coat a 13 x 9-inch baking pan with no-stick cooking spray. Stir together flour, baking powder, baking soda and salt in small bowl.
BEAT butter and 3/4 cup sugar in large bowl with electric mixer on medium speed until light and fluffy. Add egg yolks and vanilla, beating until blended. Alternately add flour mixture and water, one third at a time, beating until blended. Spread evenly in prepared pan. Spread preserves over batter.
WASH mixer beaters before beating egg whites. Beat egg whites in medium bowl with electric mixer on high speed until soft peaks just begin to form. Gradually beat in remaining 1/2 cup sugar until stiff peaks form. Spread meringue mixture over preserves layer.
BAKE 20 minutes. Sprinkle evenly with walnuts. Bake an additional 20 to 25 minutes or until meringue is a light golden color and edges are set. Cool completely in pan on wire rack before serving.