Milk and lemon juice/vinegar: Buttermilk can also be used as a substitute (if you use buttermilk, don’t add any additional lemon juice or vinegar) The acidity brings out a slight tang which perfectly matches with the sweetness of the muffins. The acid also tenderizes the gluten, allowing for a more soft texture. Regular milk would also work as a replacement
Sifting the dry ingredients: Although not completely necessary, sifting is usually always recommended in baking. Especially since cocoa powder tends to clump together, using a sift allows you to break apart the lumps of cocoa, allowing it to be better distributed.
Coffee: Coffee may be substituted for hot water, but it is highly recommended to use coffee even if you don’t like the taste! There will only be a small quantity of coffee in this batter, which is used to intensify the chocolate flavor (you can’t taste it at all!)
Do not overmix: As with the majority of baked goods, do not overmix the batter! If overworked, the gluten in the flour will develop strands, making the muffins dense and heavy.
Do not overbake: Remove the muffins from the oven as soon as a toothpick comes out clean when inserted! If it spends longer than is needed, the muffins are going to turn out dry