Peanut Butter Swirls

1/2 cup (1 stick) unsalted butter, at room temperature
1 cup sugar
1/2 cup peanut butter
1 large egg
2 tablespoons milk
1-1/4 cups all-purpose flour
1/2 teaspoon salt
1/2 teaspoon baking soda
6 ounces semi-sweet chocolate chips

Cream butter and sugar. Beat in peanut butter, egg, and milk. Set aside. Sift flour, salt, and baking soda into a separate bowl. Stir flour mixture into butter mixture. Combine well. Chill several hours. Place dough on floured wax paper and roll into a 15 by 8 by 4-inch rectangle. Melt chocolate in top of a double boiler. Cool slightly. (Chocolate should not be so hot that it melts dough, nor should it be so cold that it will not spread.) Spread chocolate on dough mixture. Using wax paper to help, roll dough up jelly-roll style. Place a baking sheet under roll to help transfer it to refrigerator. Chill 8 hours or overnight. Preheat oven to 375°. Slice roll into 1/4-inch slices with a very sharp knife. Bake 8 minutes on an ungreased baking sheet. Cool cookies slightly before removing from baking sheet. Cool on wire racks. YIELDS 3 to 4 DOZEN COOKIES.