Steps
1. Preheat your oven to 350°F (175°C) and grease a small baking dish.
2. In a bowl, mix the melted butter, granulated sugar, brown sugar, and vanilla until combined.
3. Add the egg and whisk until smooth.
4. In a separate bowl, whisk together the flour, cocoa powder, baking powder, and salt.
5. Gradually add the dry ingredients to the wet mixture, stirring until just combined.
6. Fold in the diced apple and ground cinnamon until evenly distributed.
7. Pour the batter into the prepared baking dish and spread it evenly.
8. Bake for 20-25 minutes, or until a toothpick inserted in the center comes out clean.
9. Let it cool for a few minutes before slicing and serving.