Preheat your oven to 375°F (190°C).

Line a 12-cup muffin tin with paper liners or lightly grease the cups.

In a medium bowl, whisk together flour, sugar, salt, and baking powder.

In a smaller bowl, combine oil, egg, milk, and vanilla extract. Whisk until smooth.

Gradually add the wet mixture to the dry mixture. Stir until just combined. Avoid overmixing, as this can make the muffins dense.

Gently fold in the blueberries using a spatula.

Mix flour and sugar in a small bowl.

Add the cold butter and use your fingers or a pastry cutter to blend until the mixture resembles coarse crumbs.

Fill the muffin cups about ¾ full with the batter.

Sprinkle the crumb topping evenly over the muffins (if using).

Bake for 20–25 minutes or until a toothpick inserted into the center comes out clean.

Allow muffins to cool in the pan for 5 minutes, then transfer to a wire rack to cool completely.

Use Fresh Blueberries if Possible: Fresh blueberries give a burst of flavor and better texture. If using frozen, do not thaw them to prevent the batter from becoming watery.

Coat Blueberries in Flour: Lightly toss the blueberries in a tablespoon of flour before folding them into the batter. This helps prevent them from sinking to the bottom.

Do Not Overmix: Mix the batter just until the ingredients are combined. Overmixing can lead to tough muffins.