Preheat your oven to 350°F (175°C) and line a 9x13 inch baking pan with parchment paper.
In a medium saucepan over medium heat, melt 12 tablespoons of unsalted butter, cooking until golden brown and nutty in aroma. Remove from heat and cool slightly.
In a large mixing bowl, whisk together the browned butter, 1 cup of loosely packed brown sugar, and 2 teaspoons of vanilla until well combined. Add in the 2 lightly beaten eggs, mixing until smooth.
In another bowl, sift together 1 3/4 cups of all-purpose flour, 1/2 teaspoon of baking powder, 1/2 teaspoon of baking soda, and 1/4 teaspoon of salt. Gradually add this dry mixture to the wet ingredients, stirring until just combined.
Gently fold in 1 cup of chocolate chips, distributing them evenly throughout the batter.
In a separate bowl, combine 2/3 cup of rolled oats, 2/3 cup of loosely packed brown sugar, 2/3 cup of all-purpose flour, and 1/4 teaspoon of ground cinnamon. Cut in 5 tablespoons of softened butter until the mixture resembles coarse crumbs.
Spread the blondie batter evenly in the prepared baking pan, then sprinkle the oatmeal cookie streusel mixture generously over the top.
Bake in the preheated oven for 25-30 minutes, or until the edges are golden brown and a toothpick inserted comes out with a few moist crumbs.
Once baked, cool in the pan for about 10 minutes before transferring to a wire rack to cool completely. Slice into squares and serve!
