1 cup (2 sticks) unsalted butter, softened
1 3/4 cup packed dark brown sugar
3/4 cup light corn syrup
2 teaspoons pure vanilla extract
5 1/3 cups oats, old-fashioned or quick-cooking

Glaze
12 ounces semisweet chocolate chips
1 cup smooth peanut butter

Preheat the oven to 350 degrees F with a rack in the center of the oven. Grease the baking pan with 1 tablespoon unsalted butter or solid vegetable shortening. Set aside.

In a large mixing bowl, beat the butter with the mixer on high speed until creamy. Gradually add the dark brown sugar and continue to beat until light and fluffy, about 2 minutes. Some small nuggets of dark brown sugar may remain in the mixture, but that is okay. Move the beaters around the bowl and scrape down the sides of the bowl with a rubber spatula several times while mixing. Lower the mixer speed to medium and beat in the corn syrup and vanilla extract, then stir in the oats with a wooden spoon. Spread the batter in the prepared pan and bake for 16 minutes. Remove from the oven and cool in the pan on a wire rack until lukewarm.

While the bars are baking and cooling, make the glaze. Place the chocolate chips and peanut butter in a small saucepan and stir over low heat until the chocolate melts and the mixture is smooth. Spread the mixture over the lukewarm bars, then cool completely or refrigerate until cold. Cut into bars when cold.

The bars can be stored in a covered tin at room temperature for up to 1 week. Cushion the bars between layers of plastic wrap or wax paper to protect the frosting. To freeze, allow them to cool completely after baking, then spread them on a baking sheet and place in the freezer for at least 3 hours.

When the bars are thoroughly frozen, stack them in freezer-strength zip-type gallon-size plastic bags for up to 3 months. Use a waterproof marker to note the contents of the bags and the date. Defrost them in the bags at room temperature for 1 to 2 hours.