Start by melting the butter. In a large mixing bowl add the melted butter and granulated sugar and whisk together until combined.
100 g butter, 100 g granulated sugar
Then add the egg, whole milk, sour cream and vanilla extract and whisk it together until combined.
1 large eggs, 50 g sour cream, 90 g whole milk, 1 teaspoon vanilla extract
Place a sieve on the bowl and sift in the all-purpose flour, baking powder, baking soda and salt together. Stir in the dry ingredients into the batter until combined.
160 g all-purpose flour, 1 ¼ teaspoon baking powder, ¼ teaspoon baking soda, ¼ teaspoon salt
In a separate bowl, add blueberries and 1 tablespoon of flour and stir to coat the blueberries with flour.
100 g fresh or frozen blueberries + 1 tablespoon flour
Add the blueberries and the extra flour and gently fold it into the batter.
Cover the bowl with some plastic wrap and let the batter rest for 1 hour at room temperature.
While the batter rests, preheat the oven to 220ºC/425ºF conventional oven. Prepare a 12-cup cupcake pan with 6 cupcake liners, only lining every other cupcake hole, as it helps them to dome up. Grease around the edge of the cupcake holes, for the muffins to release easily. Then make the streusel.