Preheat oven to 350 degrees. With an electric mixer, cream together ½ cup butter and powdered sugar. Add flour and mix well. Dough will be very light and delicate to the touch. Pat dough firmly into the bottom of an ungreased approximately 9 inch square or rectangular pan. Bake at 350 degrees for about 12 minutes or until lightly golden brown. Meanwhile, in a medium saucepan, melt 3 tablespoons of butter, brown sugar, lemon juice and water. Bring to a boil stirring constantly. Remove from heat and stir in nuts and vanilla. (Note: Nuts can be a combination like walnuts, pecans and slivered almonds totaling 1 cup.) Spread nut mixture evenly over pre-baked shortbread crust. Bake for 15 minutes. Cool slightly, and cut into bars while still warm. (If freezing, slice bars first before freezing.) Makes approximately 2 dozen small bars.