Mix together the black cocoa powder and the dark rum. Let sit in an airtight container for at least an hour or a couple of days.
1 bar spoon black cocoa powder, ¾ ounce dark rum
When you're ready, strain out some of the black cocoa powder by passing the mixture through a fine mesh strainer and/or cheesecloth.
Add the simple syrup, white rum, coconut rum, and heavy cream to a cocktail shaker.
1¼ ounce simple syrup, 1 ounce white rum, ½ ounce coconut rum, 1 ounce heavy cream
Add ice to the shaker and shake well, about 15-20 times.
Strain over fresh ice into your serving glass.
Carefully pour the black cocoa rum infusion over top of the drink by slowly pouring it onto the back of a bar spoon, so it will float at the top of the drink.
Serve immediately.
Notes
You can reduce the calories in this drink by using half and half instead of cream, or by using a sugar substitute, like allulose, in your simple syrup instead of sugar.