Preheat oven to 200°C/400°F, place the oven rack in the middle of the oven. Line a 12-cup muffin pan with cupcake liners.
If using chocolate bars, roughly chop the chocolate and keep aside 2-3 large chunks per muffin, about 36 chunks. Set aside.
170 grams white chocolate chips, 170 grams chocolate chips
In a large mixing bowl, sift together the flour, cocoa powder, and baking powder. Add in the salt and sugar. Whisk through to combine. Add the chocolate chunks or sweet chocolate chips and stir through to combine.
310 grams plain flour, 45 grams unsweetened cocoa powder, 150 grams white granulated sugar, ½ teaspoons salt, 3 teaspoons baking powder
In a separate mixing bowl, jug, or saucepan, melt the butter and allow it to cool. Add in the rest of the wet ingredients (milk, yogurt or sour cream, eggs, and vanilla) and whisk together until well combined.
125 grams butter, 2 eggs, 125 grams Greek yogurt, 125 grams milk, 3 teaspoons vanilla extract
Pour the wet ingredients into the dry ingredients and mix until just combined. *Note 3
Divide the muffin batter evenly between the 12 cupcake liners. Filling the liners to the top. Top the muffins with the extra chocolate chunks or chocolate chips.
Bake the muffins at 200°C/400°F for 5 minutes. Then turn down the oven to 180°C/350°F and bake for a further 12 to 18 minutes until just baked. Allow the muffins to cool in the tin for 5 to 10 minutes before turning them out onto a wired rack.
Allow the muffins to cool for 10 to 15 minutes before serving.