1. Preheat your oven to 375 F (190 C). Line a 12-cup muffin tin with paper liners or grease it well with non-stick spray.

2. In a large mixing bowl, whisk together the 1 ½ cups all-purpose flour, ½ cup granulated sugar, 2 teaspoons baking powder, and ½ teaspoon salt. Make sure there are no lumps and the ingredients are evenly distributed.

3. In a separate medium bowl, whisk together the large egg, ½ cup milk, ¼ cup vegetable oil, and 1 teaspoon vanilla extract until well combined and smooth.

4. Pour the wet ingredient mixture into the dry ingredient mixture. Stir gently with a spoon or spatula until just combined. The batter should be slightly lumpy; do not overmix, as this can lead to tough muffins. Gently fold in the blueberries.

5. In a small bowl, combine the ¼ cup all-purpose flour and ¼ cup granulated sugar. Add the cold, cubed 2 tablespoons of unsalted butter. Using your fingertips or a pastry blender, cut the butter into the flour and sugar mixture until it resembles coarse crumbs.

6. Divide the muffin batter evenly among the 12 prepared muffin cups, filling each about two-thirds full. Sprinkle a generous amount of the crumble topping over the top of each muffin.

7. Place the muffin tin in the preheated oven and bake for 20-25 minutes, or until the tops are golden brown and a wooden skewer inserted into the center of a muffin comes out clean.

8. Once baked, remove the muffins from the oven and let them cool in the muffin tin for 5 minutes before transferring them to a wire rack to cool completely. Enjoy them warm or at room temperature.