Start by making the apple filling. In a sauté pan over medium heat, melt the butter.
4 Tbsp salted butter
Once melted, add the diced apples (¼-inch pieces), brown sugar, cornstarch, and cinnamon. Cook, stirring frequently, for 8–10 minutes, until the apples are fork-tender. Set aside.
4 cups diced apples , 1/3 cup (81g) brown sugar, 1 Tbsp corn starch, 1 1/2 tsp cinnamon
Preheat the oven to 350°F (180°C) and line a 9×13 pan with parchment paper, leaving overhang for easy removal.
To make the cookie dough, cream the butter and sugars in a stand mixer until light and fluffy. This will take about 2 minutes.
1 cup (222g) salted butter, softened, 2/3 cup (162g) brown sugar, 1/3 cup (76g) granulated sugar
Scrape down the bowl, and add the eggs, vanilla, and molasses; mix until combined.
2 eggs, 2 tsp vanilla extract, 2 tsp molasses
Once combined, add the flour, baking powder, and salt and mix just until a dough forms.
3 1/4 cups (494g) all-purpose flour, 2 tsp baking powder, 1/2 tsp salt
Remove 1 cup of dough and set aside for the crumble topping. Press the remaining dough evenly into the prepared pan.
Spread the apple filling evenly over the dough.
Make the crumble by adding the oats, brown sugar, and cinnamon to the reserved cookie dough. Mix until combined.
1 cup reserved cookie dough, 1/2 cup (50g) rolled oats, 2 Tbsp brown sugar, 1/2 tsp cinnamon
Crumble the topping over the apples, leaving some filling visible.
Bake for 22–26 minutes, or until the topping is lightly browned.
Cool completely before removing from the pan and slicing.
Store in an airtight container at room temperature for 3–5 days. This is best enjoyed the first day for the crunchiest topping.