Honestly, it sort of worked, but there wasn't quite as much chocolate flavor as I hoped for. And I'm not sure that adding more is the right answer. Now, after the fact, I'm thinking that perhaps heat is the right answer. Maybe next time I'll make a simple syrup and infuse the coffee into that.
Meanwhile, this has strong coffee flavor and a very mild chocolate flavor. It's really good, but not exactly what I was looking for. But that's okay. More experimenting is fun.
Coffee and Chocolate Liqueur
2 cups water
2 cups sugar
2 cups ground coffee
1/4 cup cocoa
Combine all ingredients in a half-gallon jar. If you like, top it off with more rum to fill the container a little more.
Shake the container to combine the ingredients and dissolve the sugar. Let it sit in a cool spot away from direct sunlight for a minimum of two weeks, or longer if you have time.
Strain the mixture through a fine metal strainer to remove the ground, strain a second time through a paper towel to remove the fine bits, and strain again through a coffee filter to remove the really fine sediment.
If it's not as clear as you like, you can strain again through a coffee filter.
Transfer to an appropriate container for storage.