Step 1: Preheat the oven to 350°F (175°C) to ensure even baking.
Step 2: Grease a muffin tin or line with paper liners to prevent sticking.
Step 3: In a large mixing bowl, whisk together the flour, baking soda, salt, and cocoa powder.
Step 4: Melt the butter in a separate bowl and let it cool slightly.
Step 5: Combine the sugar, melted butter, eggs, and vanilla extract in another bowl and whisk until smooth.
Step 6: Stir in the sour cream into the wet mixture until fully incorporated.
Step 7: Gradually add the dry ingredients to the wet mixture, stirring gently until just combined.
Step 8: Fold in the dark chocolate chips, reserving some for topping.
Step 9: Fill each muffin cup about 2/3 full with batter and top with reserved chocolate chips.
Step 10: Bake for 18-22 minutes or until a toothpick comes out clean.
Step 11: Let cool in the tin for 5 minutes before transferring to a wire rack.