Double Chocolate Muffins

Ingredients
1/2 cup unsalted butter, softened
1 cup sugar
2 teaspoons vanilla
2 teaspoons baking powder
2/3 cup unsweetened cocoa powder
1/2 teaspoon salt
2 eggs
1 cup milk
1/3 cup sour cream
2 cups all-purpose flour
1 1/2 cups chocolate chips
Preparation method