Preheat your oven to 375ºF. Prepare your muffin tin with liners or a light greasing to prevent sticking.

In a bowl, mix together sugar, egg, oil, milk, and vanilla until thoroughly combined.

Combine flour, baking powder, and salt, then gently fold into the wet ingredients until just mixed. Avoid over-mixing.

Add sour cream, mixing until evenly distributed. Gently fold in most of the blueberries, reserving some for topping.

Divide the batter evenly into the muffin tin. If making taller muffins, fill to the top; for smaller ones, fill about 2/3 full. Top with remaining blueberries and sprinkle with optional sugar.

Bake for approximately 25 minutes, adjusting if using frozen berries or making fewer muffins. Check for doneness with a toothpick.

For larger muffins, bake at 350ºF for around 40 minutes.

To keep muffins moist, store them in a sealed container while still warm.