You could add a little more sour cream to the recipe, resulting in a slightly looser batter, but then the muffins will spread out more and you will lose the dome, which will be flatter. Increase the sour cream to 170 mL (2/3 cup) instead of 125 mL (1/2 cup) and also add a little more sugar, using 250 grams (1-1/4 cups) instead of 200 grams (1 cup) will make a more cake-y muffin that is very moist.