For shortbread: Preheat oven to 350 degrees. Generously butter 9-inch square baking dish. Cream butter and sugar in processor unitl light and fluffy. Add flour and process just until blended; do not form ball. Pat into bottom of prepared dish. Bake until shortbread is light brown, about 25 minutes.
For topping: Beat sugar, maple syrup and butter to blend. Beat in egg and vanilla. Pour over shortbread. Sprinkle evenly with walnuts. Bake until topping is set, about 25 minutes. Cool on rack. Cut into 1-1/2" squares. Store in airtight container.
