In a small bowl, stir the cocoa powder in hot water till completely mixed. Keep aside to cool.
In another bowl, whisk together the flour, chocolate chips, baking powder and salt.
Preheat the oven to 180°C.
Lightly butter the muffin tray or place paper liners in each.
Using an electric/hand mixer, beat the butter and the sugar together till light and fluffy. Then add the eggs one at a time and beat till smooth.
Add the vanilla extract and continue to beat till smooth.
Next, add the flour mixture and beat lightly till just incorporated.
Add the cooled cocoa mixture and stir lightly till smooth.
Pour the batter into the muffin cups, filling it upto two-thirds of each cup.
Bake for 15-20 minutes till the cakes have risen and springy; check after 15 minutes with a tooth pick and bake accordingly. (If cooked, the toothpick should come out clean). Do not overbake or you end up with dry cupcakes which is not good at all.
Leave on a wire rack to cool and then decorate with the buttercream.