1. Preheat your oven to 400°F (204°C). Prepare a 12-cup muffin pan by lining with paper liners or lightly greasing the cups.
2. In a large bowl, whisk together the flour, sugar, baking powder, and salt until evenly combined.
3. In a separate bowl or measuring jug, beat the eggs until smooth. Add melted butter or oil, milk, and vanilla extract, whisking until thoroughly combined.
4. Pour the wet ingredients into the dry mixture and gently stir until just combined. The batter will be thick and slightly lumpy—avoid overmixing to keep muffins tender.
5. Carefully fold in the blueberries, using frozen berries if desired to prevent color bleeding.
6. If using the streusel topping, mix the flour, brown sugar, granulated sugar, cinnamon, and salt in a bowl. Cut in the cold butter with a pastry cutter or fork until the mixture resembles coarse crumbs.
7. Divide the batter evenly among the muffin cups, filling them nearly full. Sprinkle about 1 tablespoon of the streusel topping on each muffin, or if skipping streusel, optionally sprinkle 1 tablespoon granulated sugar on top for a crackly crust.
8. Bake for 18 to 22 minutes or until the muffin tops are golden and a toothpick inserted into the center comes out clean or with moist crumbs. For mini muffins, reduce baking time to 9–11 minutes accordingly.
9. Allow muffins to cool in the pan for a few minutes, then transfer to a wire rack to cool completely before storing.