Preheat the oven to 400℉/205℃. Line your jumbo muffin tin with liners and set aside.
In a small bowl, make the streusel. Combine flour, white sugar, and cold cut up butter. Using your hands, mix it together until combined into a crumbly, streusel mixture. Set aside while you make the muffin batter.
3/4 cup all-purpose flour, 5 tbsp butter, 1/3 cup white sugar
In a medium-sized bowl, add the flour, baking powder, baking soda, and salt. Mix together and set aside.
2 cups all purpose flour, 2 tsps baking powder, 1 tsp baking soda, 1/2 teaspoon salt
In a large-sized bowl, add the white sugar and lemon zest. Using your fingers, rub them together for 2 minutes. This is going to release the lemon oils and fragrance.
1 cup white sugar, Zest of 1 lemon
In the same bowl, add oil, eggs, sour cream, vanilla, and lemon juice. Whisk together until combined and smooth.
1/2 cup oil, 2 large eggs, 1 tbsp lemon juice, 1 tbsp vanilla, 1 cup sour cream
Add in the dry ingredients and using a spatula, slowly fold until a soft muffin batter forms. Mix until JUST combined.
In the bowl with the blueberries, add 1-2 tsps of flour and gently toss around until evenly coated. The light coating of flour will absorb any extra liquid it may have before adding it into the mixing bowl.
1 1/2 cups fresh blueberries
Add the blueberries and mix in a few times.
Evenly divide the batter between the muffin liners. Make sure they are about 3/4s of the way full max, no more or else they will overflow. This recipe made 8 jumbo muffins for me, but depending on the size of your jumbo pan it can make more or less.
Sprinkle streusel on top.
Bake the muffins at 400°F for 22-27 minutes. The muffins are ready when they bounce back at touch or a toothpick inserted comes out clean.
Cool, and enjoy!