1. Prepare Your Muffin Tin and Oven: Start by preheating your oven to 375 F (190 C). Line a 12-cup muffin tin with paper liners or grease it well with non-stick spray. This ensures your muffins won’t stick and makes for easy cleanup.

2. Whisk Dry Muffin Ingredients: In a large mixing bowl, combine the 1 1/2 cups all-purpose flour, 1/2 cup granulated sugar, baking powder, and salt. Use a whisk to thoroughly combine these dry ingredients, ensuring the leavening agents are evenly distributed.

3. Combine Wet Muffin Ingredients: In a separate medium bowl, whisk together the egg, milk, melted butter, and vanilla extract until well combined. The mixture should look smooth and slightly pale.

4. Gently Mix Wet and Dry: Pour the wet ingredients into the dry ingredients. Stir with a spatula or wooden spoon just until the flour mixture is moistened. It’s crucial not to overmix; a few lumps are perfectly fine. Overmixing can lead to tough muffins.

5. Fold in Blueberries: Gently fold in the blueberries. If using frozen blueberries, do not thaw them. Stir just enough to distribute them throughout the batter without crushing them.

6. Prepare the Crumble Topping: In a small bowl, combine the 1/4 cup all-purpose flour and 1/4 cup granulated sugar for the topping. Add the cold, cubed butter. Using your fingertips or a pastry blender, work the butter into the flour and sugar until the mixture resembles coarse crumbs.

7. Fill Muffin Cups and Add Topping: Divide the muffin batter evenly among the 12 prepared muffin cups. Sprinkle a generous amount of the crumble topping over the top of each muffin.

8. Bake Until Golden: Bake for 20-25 minutes, or until the muffins are golden brown and a wooden skewer inserted into the center of a muffin comes out clean. Let them cool in the muffin tin for a few minutes before transferring them to a wire rack to cool completely.