In a bowl, dissolve cocoa powder in boiling water.

In a saucepan, bring sugar and water to a simmer, stirring until sugar is dissolved.

Add sugar syrup to cocoa syrup. Strain through a fine-mesh sieve into a jar with a lid. Add the vodka, cover and refrigerate over night. (It will keep up to a month, but will lose potency over time.).

To serve, stir well and strain again through a fine-mesh sieve into a martini glass, 2/3 full. Top with lightly-sweetened whipped cream, or float heavy cream on top. (Hold a spoon with the bowl down and the edge almost touching the liqueur in the glass. Pour the cream slowly over the back of the spoon until a layer 1/8 to 1/4 inch deep floats on top of the liqueur.) Garnish with cocoa nibs or chocolate shavings.