Preheat your oven to 350°F (175°C). This ensures that your muffins bake evenly.
Line your muffin tin with baking cups or grease it with butter or non-stick spray to prevent the muffins from sticking.
In a large mixing bowl, whisk together the all-purpose flour, baking soda, salt, and cocoa powder until well-combined. Set aside.
In another bowl, beat the softened unsalted butter and sugar together until light and fluffy. This should take about 3-4 minutes.
Add the eggs one at a time, mixing well after each addition. Stir in the vanilla extract until fully incorporated.
Mix in the sour cream until the batter is smooth and creamy. This will help keep the muffins moist.
Gradually add the dry mixture to the wet mixture, stirring until just combined. Be careful not to overmix; a few lumps are okay.
Gently fold in the semi-sweet chocolate chips and 1 cup of the dark chocolate chips, ensuring they are evenly distributed throughout the batter.
Using a spoon or an ice cream scoop, fill each muffin cup about 2/3 full with batter. Sprinkle the remaining dark chocolate chips on top for extra chocolatey goodness.
Bake in the preheated oven for 18-20 minutes or until a toothpick comes out clean when inserted in the center of a muffin. Keep an eye on them to avoid overbaking!
Once baked, remove the muffins from the oven and let them cool in the pan for about 5 minutes. Then transfer them to a wire rack to cool completely.