Step 1: Prepare the Muffin Pan

1. Preheat your oven to 180°C / 350°F.
2. Line a muffin pan with paper liners or lightly grease the pan with oil or butter.
3. For a bakery-style look, fill the muffin cups almost to the top. This helps the muffins rise nicely and gives them a fuller shape.

Step 2: Mix the Wet Ingredients

1. In a large mixing bowl, add the eggs, sour cream or yogurt, milk, vanilla extract, and vegetable oil.
2. Whisk everything together until smooth and well combined.
3. The sour cream or yogurt helps create a soft, tender crumb. The oil keeps the muffins moist even after they cool.

Step 3: Mix the Dry Ingredients

1. In a separate bowl, add the flour, cocoa powder, granulated sugar, baking powder, baking soda, and salt.
2. Mix everything well so the cocoa powder, flour, and raising agents are evenly combined.
3. This step helps the muffins rise evenly and gives a smooth chocolate flavor throughout the batter.

Step 4: Combine Wet and Dry Ingredients

1. Add the dry ingredients into the wet ingredients.
2. Use a spatula or whisk to mix gently until just combined. Do not overmix the batter.
3. The batter should look thick, rich, and chocolatey. A few small lumps are okay. Overmixing can make muffins dense instead of soft.

Step 5: Add Chocolate Chips and Chopped Cooking Chocolate

1. Fold in the semi-sweet chocolate chips.
2. Then add the 25g chopped cooking chocolate and gently fold it into the batter.
3. The chocolate chips hold their shape better, while the chopped cooking chocolate melts slightly into the muffins. This gives the muffins a richer chocolate texture.
4. You can save a few chocolate chips to sprinkle on top before baking. This makes the muffins look more bakery-style.

Step 6: Fill the Muffin Cups

1. Scoop the batter into the prepared muffin pan.
2. Fill each muffin cup about ¾ full or almost to the top if you want taller muffins.
3. Sprinkle extra chocolate chips on top if you like.

Step 7: Bake the Muffins

1. Bake the muffins at 180°C / 350°F for about 18 to 22 minutes.
2. The baking time can vary depending on your oven and muffin size. Start checking around 18 minutes.
3. Insert a toothpick into the center of a muffin. If it comes out with moist crumbs, the muffins are ready. If it comes out with wet batter, bake for a few more minutes.
4. Do not overbake, because chocolate muffins can become dry.