In a bowl, take flour, sugar, cocoa powder, baking powder, baking soda and salt.

Mix them well. If your cocoa powder has many lumps then don't forget to sift it.
In another bowl add oil, vanilla, vinegar and water. Beat it very well.

Then add dry mixture to wet
Whisk it till it gets incorporated well. Do not over beat it.

Divide the batter in to prepared 12 muffins cups.
Sprinkle some chocolate chips or place a piece of chocolate on top.

Bake into pre-heated oven for 20-22 minutes. Check by inserting a toothpick into the center and if it comes out clean then muffins are ready.

Remove it from the oven; let them cook in a pan for 5 minutes.
Then remove them on cooling rack to cool completely.