Beat together butter or margarine and sugar until mixture is fluffy. Add vanilla and egg, blending well. Whisk together flour, baking soda and salt; stir in nuts. Gradually add to butter mixture; beat well. Place dough on wax paper; shape into 2 long rolls, 2 inches in diameter. Chill overnight, or, if desired, wrap and freeze for future use. Slice dough 1/4-inch thick; bake on ungreased baking sheets at 400 degrees for about 7 minutes, or until lightly golden. Cool. Combine powdered sugar and enough water to make a thin frosting. Frost cookies. Before frosting has set, make circles using the black decorator tube frosting. Add a dot in the center if desired. Use the toothpick to pull the black frosting from the center to the edges making the frosting look like spiderwebs. Let frosting dry before stacking.