Add the blueberries, lemon, sugar and vanilla to a medium sized sauce pot. Cook over medium heat until blueberries begin to boil. Turn down to a simmer, stirring and smashing occasionally for 10 minutes
Remove from heat and set aside to cool
Streusel and Streusel Crunch
Preheat the oven to 350°F
Add room temp butter, sugar and flour to a medium mixing bowl, smash together and mix with a fork or your hands until fully combined. Set half aside
Spread the other half evenly on a parchment paper lined baking sheet and bake for 7-10 minutes until streusel is a light - medium brown color. Set aside to cool
Mix the wet ingredients
Preheat the oven to 425° and grease 10 muffin liners
Melt butter and let cool in a medium sized bowl. Add the sugar and stir until combined
Add the eggs, vanilla, buttermilk and sour cream. Stir until fully combined
Mix the dry ingredients
In another medium bowl, whisk the flour, baking powder and salt
Combine wet and dry ingredients
Add the dry ingredients into the wet and fold together just until mixed. DO NOT OVER MIX
Layering the muffins
Scoop the muffin batter into the greased muffin liners. About ⅓ full in each liner
Sprinkle the crunchy streusel over the top of all the muffins evenly
Add a layer of blueberry vanilla sauce over the top of the crunchy streusel
Top off the muffins with the rest of the batter. I like to fill mine to the top to help get a bigger muffin top
Top the muffins with the uncooked streusel and demerara sugar
Bake the muffins at 425°F for 5 minutes. Turn the oven down to 350°F and bake for an additional 15-20 minutes. Insert a toothpick to check doneness, the toothpick should come out clean or with only a few crumbs. Be sure not to overbake. Enjoy the muffins warm or cool!