3Whisk for 2-3 minutes or until pale in color. Whisk in melted butter until well combined.
4Add milk mixture, vanilla bean paste, sour cream, and oil. Mix until well combined.
5Combine dry ingredients: flour, baking powder, baking soda, and salt. Reserve 2 tablespoons of dry mixture to coat mixed berries.
6Pour wet mixture into dry ingredients and mix until just combined.
7Toss mixed berries in reserved dry mixture and add to batter. Do not overmix.
8Let the batter sit at room temperature for 30 minutes.
9Prepare the baking tin by lining it.
10Divide batter evenly among muffin cups. Add more berries on top if desired.
11Bake for 5 minutes, then reduce oven temperature to 375°F (190°C). Continue baking for 15 minutes or until a toothpick inserted in the center comes out clean.
12Allow muffins to cool in tin for 5 minutes before transferring to a wire rack to cool completely.