For the oatmeal layer-
-2 1/2 cups all-purpose flour
-1 tsp baking soda
-1 tsp salt
-1/2 tsp ground cinnamon
-2 sticks unsalted butter, at room temperature
-2 cups (packed) brown sugar
-2 large eggs
-2 tsp pure vanilla extract
-3 cups old-fashioned (rolled) oats
-1 cup salted peanuts, coarsely chopped
For the chocolate layer-
-14 oz can sweetened condensed milk
-2 cups semi-sweet chocolate chips
-2 tbsp unsalted butter
-1/4 tsp salt
-1 tsp pure vanilla extract
-1 cup moist, plump raisins (I used 1 box raisinettes)
-3/4 cup coarsely chopped peanuts, preferably salted
1. Preheat oven to 350F. Butter a 9 x 13 pan and place the pan on a baking sheet.
2. Whisk together the flour, baking soda, salt, and cinnamon. Set aside.
3. With a stand mixer, beat 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 is added.
4. Beat in the vanilla. Once the mixture is light and fluffy, reduce the mixer speed to low and add the dry ingredients, mixing just until they disappear.
5. Still on low speed, or by hand, stir in the oats and chopped peanuts.
6. Set aside 1 1/2-2 cups of the mixture, and press the remaining dough evenly into the buttered pan.
7. Set a heat-proof bowl over a saucepan of simmering water. Pour 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 from the bowl from the pan of water and stir in the vanilla, raisins, and peanuts. (I did not add the raisinettes here because I thought they'd melt. Instead, I sprinkled them over the chocolate after I spread it on the crust)
8. Pour the warm chocolate over the oatmeal crust, and scatter the remaining oatmeal mixture over the top (don't try to spread, just kind of drop chunks of the batter onto the chocolate layer).
9. Bake for 25-30 minutes, or until the topping is golden brown and the chocolate layer is dull and coming away from the sides of the pan.
10. Transfer the baking pan to a rack and cool for 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 and refrigerate for at least 1 hour before cutting.