Preheat the oven to 425°F. Spray nonstick spray into 12 standard-size muffin tins or line them with cupcake liners.
Mix together the melted butter and sugar in a large bowl, then mix in the egg and vanilla extract. Then mix in the sour cream and milk one at a time until it’s fully incorporated.
Add the flour, baking powder, and salt to the mixture. Mix until it’s almost fully incorporated (a few specks of flour are okay). Then fold in the blueberries and chocolate chips into the batter.
Scoop batter into the muffin tins, filling them ⅞th of the way up.
Bake for 5 minutes at 425°F, then turn the oven down to 375°F and bake for an additional 18-20 minutes (20-22 if using frozen blueberries). Once the muffins are golden brown remove them from the oven and allow them to cool for 10 minutes.
Notes
You can substitute ½ cup of greek yogurt in place of the sour cream, but I think the sour cream gives the muffins a better flavor.Substitute ¾ cup of frozen blueberries for ¾ cup of fresh blueberries and cook the muffins for 2 minutes longer.