Brown Both Sticks of Butter

1. Add two sticks of unsalted butter to a small saucepan over medium heat. Stir until the butter is fully melted. Continue to stir, scraping the bottom of the pot frequently, as the mixture starts to bubble and spit for a few minutes, then foam as darkened bits appear near the bottom of the pot. Once the mixture turns an amber brown color and smells like caramel, remove it from the heat and pour it evenly into two small bowls.

Make the Streusel

1. In a medium mixing bowl mixing bowl, add one of the melted and browned sticks of butter (one of the two bowls you’ve just poured the butter into; keep the other set aside).
2. Add 1/3 cup brown sugar, 2 tbsp maple syrup, and 1/4 tsp salt. Whisk to combine.
3. Add 1 cup all-purpose flour and 1/3 cup oats to the mixture and stir it all together. Use your hands to squeeze handfuls of the mixture together so that clumps form; it should look like a crumble topping. Set aside.

Make the Batter

1. Preheat the oven to 350 F. Grease an 8×8 baking pan with butter and line it with parchment paper.
2. Chop your apples into small pieces.
3. To a large mixing bowl, add the remaining stick of melted and browned butter.
4. To the butter, add 1/4 cup brown sugar and 1/4 cup white sugar. Whisk to combine.
5. Add the eggs and vanilla and whisk again to combine.
6. Add the oat flour and all-purpose flour, salt, and baking powder, and use a spatula to gently fold the mixture together. Overmixing the flour can cause more gluten to develop in the flour, which can make the bars turn out tough rather than tender, so be gentle when incorporating the flour.
7. Add the chopped apples and fold them into the batter.
8. Spread the batter into the prepared baking pan. Sprinkle the crumble mixture evenly over the top of the batter.
9. Bake for 30-40 minutes, until a toothpick inserted into the center comes out clean.