For the oatmeal layer:
2½ cups all-purpose flour
1 teaspoon baking soda
1 teaspoon salt
½ teaspoon ground cinnamon
2 sticks (8 ounces) unsalted butter, at room temperature
2 cups (packed) brown sugar
2 large eggs
2 teaspoons pure vanilla extract
3 cups old-fashioned (rolled) oats
1 cup salted peanuts, coarsely chopped
For the chocolate layer:
14-ounce can sweetened condensed milk
2 cups (12 ounces) semi-sweet chocolate chips
2 tablespoons unsalted butter
¼ teaspoon salt
1 teaspoon pure vanilla extract
1 cup moist, plump raisins (dark or golden)
¾ cup coarsely chopped peanuts, preferably salted
Center a rack in the oven, and preheat oven to 350 degrees Fahrenheit. Butter a 9-by-13-inch baking pan, and place the pan on a baking sheet.
To make the oatmeal layer:
Whisk together the flour, baking soda, salt and cinnamon.
Working with a stand mixer, preferably fitted with a paddle attachment, or with a hand mixer in a large bowl, beat the butter on medium speed until it is soft and creamy. Add the brown sugar and beat for 2 minutes, then add the eggs, one at a time, beating for a minute after each egg goes in. Beat in the vanilla. The mixture should be light and fluffy. Reduce the mixer speed to low and add the dry ingredients, mixing just until they disappear. Still on low speed, or working by hand with a rubber spatula, stir in the oats and chopped peanuts.
Set aside 1½ to 2 cups of the mixture, then turn the remaining dough into the buttered pan. Gently and evenly press the dough over the bottom of the pan. Set aside while you prepare the next layer.
To make the chocolate layer:
Set a heat-proof bowl over a saucepan of simmering water. Put the condensed milk, chocolate chips, butter, and salt in the bowl and stir occasionally until the milk is warm and the chocolate and butter are melted. Remove the bowl from the pan of water and stir in the vanilla, raisins (if using), and peanuts.
Pour the warm chocolate over the oatmeal crust, then scatter the remaining oatmeal mixture over the top. Don’t try to spread the oatmeal, and don’t worry about getting the topping even — this is fun, remember?
Bake for 25 to 30 minutes, or until the topping is golden brown and the chocolate layer is dull and starting to come away from the sides of the pan. Transfer the baking pan to a rack and cool for about 2 hours.
Run a blunt knife between the edges of the cake and the pan, and carefully turn the cake out onto a rack. Turn right side up, then refrigerate for at least 1 hour before cutting.
Cut into 32 rectangles, each roughly 2¼ by 1½ inches. Makes 32 bars.
Serving: I think these are best served cold from the fridge, although my husband likes them straight from the freezer, cut into slivers. Before you chill the bars, though, have one — you might find you like them best at room temperature, in which case you’re lucky: You can start enjoying them sooner.
Storing: Wrapped well, these will keep for about four days at room temperature, 1 week in the refrigerator, or up to two months in the freezer.