Preheat the oven to 350°F and line a small baking pan with parchment paper.
Melt 3 tablespoons of the butter in a mixing bowl. Stir in the granulated sugar until combined.
Add the egg and mix until smooth. Stir in the cocoa powder, 1/4 cup flour, and salt until no dry streaks remain.
Spread the batter evenly in the prepared pan. Scatter the diced apple over the top.
Drizzle the caramel sauce over the apples.
In a separate bowl, mix the rolled oats, brown sugar, remaining 2 tablespoons flour, and remaining 1 tablespoon butter until crumbly. Sprinkle this mixture evenly over the caramel layer.
Bake for 25 to 30 minutes until the crumble turns golden and a toothpick inserted in the center comes out with a few moist crumbs.
Cool in the pan before slicing into four pieces.