Chocolate Oat Bars

1 cup (2 sticks) butter
1/2 cup firmly packed light brown sugar
1 tsp vanilla extract
3 cups quick-cooking oats
1 cup semisweet chocolate chips
1/2 cup crunchy or creamy peanut butter

Grease a 9-inch square baking pan. Melt butter in large saucepan over medium hear. Add brown sugar and vanilla; mix well.
Stir in oats. Cook over low heat until ingredients are well blended, about 2-3 minutes. Press half the mixture into prepared pan. Use back of large spoon to spread mixture evenly.
Meanwhile, melt chocolate chips in small heavy saucepan over low heat, stirring occasionally. Stir in peanut butter. Pour chocolate mixture over oat mixture in pan; spread evenly with knife or back of spoon. Crumble remaining oat mixture over chocolate layer, pressing down gently. Cover and refrigerate until set, about 2-3 hours, or overnight.
Bring to room temperature before cutting into bars. (Bars can be frozen; let thaw about 10 minutes or more before serving.)