These muffins really are big and fat. If you prefer smaller ones, fill the muffin cups only halfway. You'll end up with maybe 16 or 18. Or use a mini-muffin pan and make lots and lots of tiny ones.
2/3 cup unsweetened cocoa powder
1 3/4 cups all-purpose flour
1 cup brown sugar
1 tsp baking powder
1 tsp baking soda
1 cup white chocolate chunks, or chips (or semi-sweet)
2 tsp vanilla
1 cup plain yogurt
1/2 cup vegetable oil
In a large bowl, have your young assistant stir together the cocoa, flour, brown sugar, baking powder and baking soda. Make sure the ingredients are evenly mixed and lump-free. Mix in the chocolate chunks or chips. All of them, please.
In a medium bowl, with an electric mixer on low speed or by hand with a whisk, beat together the eggs, vanilla, yogurt and vegetable oil. Pour the egg mixture into the flour mixture, and stir just until evenly combined - don't over-stir the batter or it will deflate. Scoop batter evenly into 12 paper-lined or well-greased muffin cups - they'll be filled nearly to the top. Bake at 425°F/220°C for 15 to 20 minutes, or until a toothpick stuck into the centre of a muffin comes out clean. Let cool for 5 minutes before trying to remove from the pan. For smaller muffins, reduce baking time by about 5 minutes.