Place the cocoa powder in a bowl and slowly whisk in the boiling water until fully incorporated.
Let cool slightly, then whisk in the buttermilk.
In a different bowl, sift together the flour, baking soda and salt.
In the bowl of a mixer fitted with the paddle attachment, beat the butter and sugars on medium until combined and fluffy.
One at a time, add the eggs, beating well after each addition. Add the vanilla with the final egg.
Reduce the speed to low and add the flour mixture in 3 additions, alternating with the cocoa-buttermilk mixture in 2 additions, starting and ending with the dry ingredients. Beat until just combined.
Divide the batter among the prepared muffin cups, filling them to almost full.
Bake the cupcakes until they are puffed and slightly springy to the touch and an inserted toothpick come out clean. About 20 minutes.
Let cool slightly, then remove the cupcakes from the pan and cool completely on wire racks.