Preheat the oven to 400°F and line a muffin tin with paper liners.
In a bowl, mix together flour, brown sugar, granulated sugar, and cinnamon for the crumble topping. Add melted butter and mix until crumbly.
In a separate bowl, whisk together the flour, baking powder, salt, and cinnamon for the muffin batter.
Beat together brown sugar, granulated sugar, and room-temperature butter until fluffy. Add eggs one by one along with sour cream and vanilla; mix well.
Gradually add half of the dry ingredients followed by milk; mix gently. Fold in diced apples.
Divide batter among muffin cups and top generously with crumble mixture.
Bake for 20 minutes until golden; allow to cool on a wire rack.
