Cream butter and sugar. Beat egg yolks with fork and add to creamed mixture. Mix well. Add rum, mix. Add flour slowly. Divide dough in half, wrap in wax paper, chill in refrigerator over night. Roll cookies thinly. Cut out with cookie cutters or with knife. Brush cookies with beaten egg. Sprinkle with finely chopped walnuts (1 cup). Bake at 375 degrees for 15 minutes. Check bottom shelf after 10 minutes. sprinkle with powdered sugar.