In a medium mixing bowl, add the flour, baking powder, baking soda and salt. Whisk to combine and set aside.
In a large mixing bowl, add the eggs and sugar. Whisk until well combined. Add the melted butter, oil, sour cream, milk and vanilla extract and whisk until combined.
Add the dry ingredients into the wet mixture and, using a spatula, fold until almost combined. Fold in the blueberries. (*See notes if adding lemon juice) Let batter rest for 20 minutes and preheat the oven to 425℉ while you wait.
While the batter rests, make your streusel. In a small microwave safe bowl or dish, add the butter and heat in 20 second increments until melted. Stir in 1/4 cup flour and 3 tbsp of sugar. You should end up with clumps. Set aside.
Line or spray 12 tins on 2 different pans, alternating so there's space between each muffin or 12 tins in one pan (**see notes). Fill each tin so it's slightly overflowing. As we've rested the batter, it won't actually spill out the sides. Then top each muffin with the streusel evenly until it's gone.
Bake at 425℉ for 7 minutes and then without opening the oven door, drop the temperature to 350℉ and bake for an additional 18-22 minutes or until they have started to turn golden on top and a toothpick comes out clean. Allow to fully cool before storing in airtight container for up to 3 days and enjoy!