Preheat the oven to 375°F (190°C). Line a muffin tin with paper liners or grease with nonstick spray.

Combine the sugar, egg, oil, milk, and vanilla in a bowl and stir until well combined.

Mix the oil, egg, milk, and vanilla in a separate bowl until combined. Pour into the dry ingredients and stir until just mixed (don’t overmix!).

Add the flour, baking powder, and salt. Stir the baking powder and salt into the flour on top of the wet ingredients. Once the baking powder and salt are well distributed in the flour, stir them into the wet ingredients until they are well mixed. Don't over-mix—stir until lumps.

Add sour cream, stirring until well distributed. I use plain or vanilla yogurt, but sour cream will work, too.

Fold in the blueberries.
Note: If using frozen blueberries, there are a few extra steps to ensure the muffins come out perfectly.
Do Not Thaw - Keep the blueberries frozen until you're ready to fold them into the batter. Thawing them can release extra moisture, turning your batter purple and making the muffins too wet.
Toss Berries with Flour- Coat the frozen berries with about 2 Tablespoons of flour before folding into the batter. This helps prevent them from sinking to the bottom.
Gently Fold In—Be extra gentle when stirring frozen blueberries into the batter to avoid streaking it - Be extra gentle when stirring frozen blueberries into the batter to avoid streaking the batter purple. A few swirls are fine, but overmixing can cause too much color bleeding.

Divide the batter evenly among the muffin cups, filling about ¾ full.

Optional Crumb Topping:
¼ cup sugar
2 tablespoons flour
2 tablespoons cold butter, cubed
½ teaspoon cinnamon
Optional: Mix sugar, flour, butter, and cinnamon with a fork until crumbly for the crumb topping. Sprinkle over the muffins.

Bake for 20-25 minutes or until a toothpick inserted in the center comes out clean.
Note: If using frozen berries, adjust the baking time. Frozen berries can slightly lower the temperature of the batter, so you may want to add 2 -3 extra minutes to the baking time. Start checking for doneness at the 22-minute mark by inserting a toothpick into the center—if it comes out clean, they're ready!

Let it cool for a few minutes before enjoying it!