Combine 2 cups flour, butter, 1/2 cup sugar and cornstarch with a pastry blender or 2 forks until mixture resembles fine crumbs. Press mixture evenly into an ungreased jelly-roll pan; bake at 350° for 18 minutes.
Beat eggs, brown sugar, remaining 1/4 cup flour, baking powder and vanilla at medium speed with an electric mixer until well blended; stir in walnuts and coconut. Spread on top of crust.
Bake at 350° for 10 minutes. Let cool in pan on a wire rack; spread with Cream Cheese Topping and bake 30 more minutes or until set. Cut into bars.
Cream Cheese Topping:
Beat first 3 ingredients at medium speed with an electric mixer until creamy; gradually add sugar, beating until blended. Makes 3 1/2 cups.