In a small bowl, combine flour, sugar, and melted, cooled butter. Use a fork to mix until crumbly. Refrigerate while you make the batter so it has time to firm up a bit
In a medium bowl, whisk together, 190g all-purpose flour, 195g cake flour, ½ tsp baking soda, 1 tbsp baking powder, ¾ tsp fine sea salt, 1 tsp cinnamon
in a large mixing bowl, whisk 6 tbsp butter (melted), ⅓ cup (80ml) vegetable oil, and 250g sugar until well incorporated, about 1 minute.
Add eggs and wet ingredients: Mix in 2 eggs, one at a time. Then add: ⅓ cup (80g) sour cream, 1 cup buttermilk, 1½ tsp vanilla extract. mix until well combined
Add the dry ingredients to the wet ingredients. Mix gently until just combined, don’t overmix.
Gently fold in the floured blueberries with a few folds. Stop mixing as soon as they’re evenly distributed.
Fill each muffin cup to the top. Don’t worry about overflow, it helps create a tall, domed muffin with crispy edges
Spoon the chilled streusel mixture over each muffin. Press it in gently so it sticks. Add a sprinkle of turbinado sugar on top if desired.