In a medium bowl, combine the cocoa powder and boiling water, stirring to dissolve the cocoa. Set aside.

In a small saucepan, combine the sugar and water and bring to a simmer over medium heat, stirring until the sugar has dissolved. Stir the sugar syrup into the cocoa mixture and add the vodka. Strain through a fine-mesh strainer into a container with a lid.

Refrigerate overnight. The liqueur win keep for several weeks, but the flavor of the vodka will weaken as time goes on.

To serve, stir the liqueur well, then strain it again through a fine-mesh strainer. Fill each shot glass two-thirds full with liqueur. Top with whipped cream or “float” unwhipped heavy cream on the liqueur. Hold a spoon over the glass, just above the liqueur with the bowl of the spoon facing down. The spoon will diffuse the cream. Slowly pour until a 1/8-to ¼-inch layer of cream rests on top of the liqueur. Sprinkle with 3 to 4 nibs.