Preheat oven to 400°F (200°C). Line a 12-cup muffin tin with liners or grease it well.

In a medium bowl, combine all streusel topping ingredients (flour, sugar, cinnamon). Cut in the cold butter with a pastry blender or your fingertips until the mixture resembles coarse crumbs. Set aside.

In a large bowl, whisk together the muffin batter ingredients: flour, sugar, baking powder, salt, and cinnamon. In a separate bowl, whisk together the wet ingredients: egg, milk, vegetable oil, and vanilla extract.

Pour the wet ingredients into the dry ingredients and stir until just combined. Do not overmix. Gently fold in the blueberries.

Divide the batter evenly among the prepared muffin cups, filling each about two-thirds full. Sprinkle the streusel topping generously over the batter in each cup.

Bake for 20-25 minutes, or until a toothpick inserted into the center of a muffin comes out clean.

Let the muffins cool in the tin for a few minutes before transferring them to a wire rack to cool completely.