Line a jumbo muffin tins with liners and set aside. Melt the butter and prepare to measure the rest of your ingredients as it cools.
1/2 cup butter
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.
5 tbsp butter cold, 1/3 cup white sugar, 3/4 cup all-purpose flour
In a medium-sized bowl, add the flour, baking powder, baking soda, cornstarch, and salt. Mix together and set aside.
2 1/2 cups all-purpose flour plus extra for dusting, 3 tsp baking powder, 1 tsp baking soda, 1/2 tsp salt, 1 tbsp cornstarch
In a large bowl (this is the bowl you'll be using for the entire bake so make sure it is large!), 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.
zest of 1 large lemon, 1 cup white sugar
Add in lemon juice, oil, melted butter, buttermilk, egg, egg yolk, and vanilla. Whisk until well combined. It’s okay if it is clumpy.
3 tbsp oil, 1 egg, 1 egg yolk, 3 tbsp lemon juice, 1 cup buttermilk*, 1 tbsp vanilla, 1/2 cup butter
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 your muffin batter between your 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 425°F for 5 minutes, then turn down to 375°F and bake for an additional 17-20 minutes. The muffins are ready when a toothpick inserted comes out dry with no wet batter.
Enjoy!!