1: Preheat the oven to 425°F (220°C) and prepare the baking tin.
2: In a mixing bowl, whisk together the melted butter and sugar until well combined.
3: Add the eggs and whisk for 2-3 minutes or until pale in color.
4: Add the milk mixture, vanilla extract, sour cream, and oil. Mix until well combined.
5: In a separate bowl, combine the dry ingredients: flour, baking powder, baking soda, and salt. Reserve 2 tablespoons of the dry mixture to coat the blueberries.
6: Pour the wet mixture into the dry ingredients and mix until just combined. Toss the blueberries in the reserved 2 tablespoons of dry mixture and add to the batter. Do not overmix and let the batter sit at room temperature for 30 minutes.
7: Make the crumb topping by mixing melted butter, granulated sugar, and brown sugar until a paste forms. Add all-purpose flour and cornstarch and mix until crumbly.
8: Divide the batter evenly among the muffin cups.
9: Bake for 5 minutes, then reduce oven temperature to 375°F (190°C) without opening the door. Continue baking for an additional 15 minutes or until a toothpick comes out clean.
10: Cool in the tin for 5 minutes before transferring to a wire rack.
