Preheat your oven to 350°F (175°C) and line a muffin tin with paper liners or grease it lightly.
In a large mixing bowl, combine 1 ½ cups flour, ½ cup granulated sugar, ½ cup brown sugar, baking powder, baking soda, salt, and 1 teaspoon cinnamon.
In a separate bowl, whisk together the melted butter, egg, vanilla extract, sour cream, and milk until smooth.
Pour the wet ingredients into the dry ingredients and stir until just combined. Do not overmix.
In another bowl, prepare the crumble topping by mixing ½ cup flour, ¼ cup brown sugar, ¼ cup granulated sugar, and 1 teaspoon cinnamon. Add the softened butter and mix until crumbly.
Scoop the muffin batter into the prepared muffin tin, filling each cup about two-thirds full.
Sprinkle the crumble topping generously over each muffin.
Bake for 18-22 minutes, or until a toothpick inserted in 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.