Preheat the oven to 350 degrees F or 180 degrees C for at least 10 minutes.
Line muffin trays with cupcake liners. Alternately you can lightly spray it using non-stick cooking oil spray. and keep it aside.
Take dry ingredients (all purpose flour, sugar, cocoa powder, baking soda and salt) in a bowl. Mix very well using wire whisk. If your cocoa powder have too many lumps, please sift it.
In another bowl, mix wet ingredients (Water, oil, vanilla extract).
Add this wet ingredients to the flour mixture.
Using wire whisk, beat it till everything is incorporated well. Do not over mix it.
Now, add vinegar and mix.
Fill the each muffin cup about 2/3 full with the batter.'
If you don't have the cupcake tray then use a small glass bowl and place the cupcake liners inside as shown in image below.
Bake into preheated oven for 15-20 minutes. Check by inserting a toothpick in the center of the cake. If it comes out clean meaning cake is ready.
Remove it from the oven, let them cool for 5-10 minutes in the pan itself. Then remove it to a wire rack to cool completely.