I slightly modified this recipe from the one I found here Diana's Desserts
Double Chocolate Muffins
3 ounces 70-80% dark chocolate (for melting)
1 cup granulated sugar
1/2 cup unsalted butter, softened
1/2 cup sour cream (9 or 18% creme fraiche)
1/2 cup milk
2 cups all-purpose flour
3/4 teaspoon baking powder
1/2 teaspoon baking soda
1/2 teaspoon salt
1 100g (standard size) bar of 70-80% dark chocolate broken into small pieces to add to muffin mixture
Preheat oven to 375 degrees F (190 C). Line muffin cups with muffin or cupcake liners. You can either use the jumbo or the standard size muffin pans. I prefer the standard size.
In a microwave-safe bowl, melt chocolate in the microwave on medium-high heat for 25 second intervals, stirring in between, until smooth. Set aside.
In a large bowl of an electric mixer, cream sugar and butter. Beat in eggs, then add sour cream and milk. In a small bowl, stir together flour, baking powder, baking soda, and salt. Pour flour mixture into butter mixture and stir together by hand. Stir in melted chocolate, then chocolate pieces.
Fill each muffin cup just to the edge of the baking cup. This will give you that nice muffin top look. Bake for about 20 minutes for standard-sized muffins or 25 minutes for giant muffins, or until a toothpick inserted into the center comes out clean. Transfer muffins to wire rack to cool completely. Muffins are best eaten fresh.
Makes 6 giant or 12 standard muffins.