Preheat the oven to 350°F (175°C) and line a muffin tin with paper liners.
In a large bowl, whisk together the flour, granulated sugar, baking powder, salt, and cinnamon.
In another bowl, combine the melted butter, egg, milk, and vanilla extract. Mix well.
Pour the wet ingredients into the dry ingredients and stir until just combined. Do not overmix.
In a small bowl, prepare the crumble topping by mixing together the flour, brown sugar, softened butter, and cinnamon until crumbly.
Spoon the muffin batter into the prepared muffin tin, filling each cup about 2/3 full. Sprinkle the crumble topping generously over each muffin.
Bake for 18-20 minutes or until a toothpick inserted into the center comes out clean.
Allow the muffins to cool in the pan for 5 minutes before transferring them to a wire rack to cool completely.