Preheat your oven to 375°F (190°C) and line a 12-cup muffin tin with paper liners.
In a large bowl, whisk together the flour, granulated sugar, baking powder, and salt.
In a separate medium bowl, whisk together the milk, vegetable oil, egg, and vanilla extract.
Pour the wet ingredients into the dry ingredients and stir until just combined.
In a small bowl, combine the brown sugar and cinnamon for the swirl.
Spoon half of the muffin batter into the prepared muffin cups.
Sprinkle a portion of the cinnamon-sugar mixture over the batter in each cup.
Spoon the remaining muffin batter over the cinnamon-sugar layer.
In a small bowl, combine flour, brown sugar, ground cinnamon, and melted unsalted butter for the crumble topping.
Sprinkle the crumble topping evenly over the muffin batter in each cup.
Bake for 18–22 minutes or until a wooden skewer inserted into the center comes out clean.
Let the muffins cool in the muffin tin for a few minutes before transferring them to a wire rack to cool completely.
