For the oatmeal layer:
2½ cups all-purpose flour
1 tsp. baking soda
1 tsp. salt
¼ tsp. ground cinnamon
1 cup (2 sticks) unsalted butter, at room temperature
2 cups light brown sugar
2 large eggs
2 tsp. vanilla extract
3 cups old-fashioned oats
1 cup salted honey roasted peanuts, coarsely chopped
For the chocolate layer:
1 (14 oz.) can sweetened condensed milk
2 cups semisweet chocolate chips
2 tbsp. unsalted butter
¼ tsp. salt
1 tsp. vanilla extract
¾ cup salted honey roasted peanuts, coarsely chopped
Preheat the oven to 350° F. Line the bottom of a 9 x 13-inch baking pan with parchment paper. Grease the sides of the pan with butter.
To make the oatmeal cookie layer, combine the flour, baking soda, salt and cinnamon in a medium bowl. Whisk to blend. In a large mixing bowl, beat the butter on medium-high speed until it is soft and creamy, about 1 minute. Add the brown sugar and beat on medium-high speed until light and fluffy, about 2 minutes. Mix in the eggs one at a time, beating well after each addition and scraping down the sides of the bowl as needed. Blend in the vanilla. With the mixer on low speed, mix in the dry ingredients just until incorporated. Mix in the oats and peanuts with a rubber spatula until evenly combined.
Reserve 1½ cups of the mixture. Place the remaining dough in the prepared pan and press over the bottom of the pan in an even layer.
To make the chocolate layer, combine the condensed milk, chocolate chips, butter and salt in a heatproof bowl set over a pan of simmering water. Stir occasionally until the mixture is warm and the chocolate chips and butter are melted. Remove from the heat and stir in the vanilla and peanuts.
Pour the warm chocolate mixture over the oatmeal cookie layer and spread evenly with an offset spatula. Crumble the remaining oatmeal cookie mixture and scatter evenly over the top of the chocolate.
Bake for 25-30 minutes, until the topping is golden brown and the chocolate layer is starting to pull away from the sides of the pan. Transfer the pan to a wire rack and let cool completely. Cut into bars as desired and serve.