Preheat the oven to 10°C.
Stir the boiling water and cocoa powder until smooth and runny. Set aside and let it cool at room temperature.
In a medium bowl, whisk together the sifted flour, baking powder, and salt.
In another bowl using an electric mixer, beat the butter and sugar until light and fluffy.
Mix in the eggs one at a time until smooth, add in vanilla extract
Scrape down the side of the bowl as needed.
Add the flour mixture and beat only until fully incorporated.
Mix in the cocoa mixture until well blended.
Fill each muffin cup about 3/4 full with the batter and bake for about 20 minutes until risen, or toothpick inserted comes out clean.