Preheat oven to 425°F/220°C. In a small bowl, combine flour, sugar, and melted butter. Whisk together with a fork until little crumbles form and place in the freezer.
2 ½ tablespoon all purpose flour, 2 ½ tablespoon granulated sugar, 1 tablespoon unsalted butter
Combine the flour, baking powder, cinnamon, and salt in a medium mixing bowl and set aside.
1 ¼ cup all purpose four, 1 ½ teaspoon baking powder, ½ teaspoon ground cinnamon, ¼ teaspoon sea salt
Whisk the granulated sugar, brown sugar, melted butter, oil, egg, and vanilla extract together in a large bowl until well combined. Add the Greek yogurt and buttermilk and whisk until smooth with no lumps.
½ cup granulated sugar, 2 tablespoon brown sugar, 2 tablespoon unsalted butter, 2 tablespoon vegetable oil, 1 large egg, ½ teaspoon vanilla extract, ¼ cup Greek yogurt, ¼ cup buttermilk
Fold the dry ingredients into the wet ingredients with a rubber spatula or large spoon until a few dry streaks of flour are left.
In a separate small bowl, coat the blueberries with an additional tablespoon of flour. Fold the blueberries into the muffin batter until no dry streaks of flour remain.
1 cup frozen blueberries
Spray a 12-cup muffin tin with non-stick baking spray if desired and place cupcake liners in every other cup. Fill each ¾ full with muffin batter. Sprinkle streusel topping and coarse sugar on top.
Bake the muffins for 5 minutes, then reduce the heat to 400°F/200°C and continue baking for 15-20 minutes or until a toothpick inserted in the center comes out clean with a few moist crumbs.
Allow the muffins to cool for 10-20 minutes before eating.