Preheat the oven to 350°F (175°C) and line your muffin tin with muffin liners or spray with non-stick spray.
In a large mixing bowl, combine the melted butter, granulated sugar, and brown sugar. Whisk until well blended.
Add the eggs, milk, vanilla extract, and sourdough starter to the mixture. Whisk until smooth and well combined.
In another bowl, mix the all-purpose flour, baking soda, and salt together. Gradually add the dry ingredients to the wet ingredients, stirring gently with a spatula until just combined. Avoid overmixing.
Fold in the chocolate chips until evenly distributed through the batter.
Divide the batter equally among the 12 muffin cups, filling each about ⅔ full.
Bake in the preheated oven for 18-22 minutes or until a toothpick inserted into the center comes out clean.
Let the muffins cool in the pan for about 5 minutes, then transfer them to a wire rack to cool completely.