Find a glass bottle or jar large enough to store the rum with the coffee beans and vanilla beans for several days. Pour the rum into the container.
Split the vanilla beans down the center with a sharp knife. This will expose the inner seeds, allowing the vanilla flavor to better infuse into the rum.
Add the split vanilla beans to the bottle with the rum.
Add the coffee beans to the bottle with the vanilla beans and rum, cover it, and give it a good shake. I use whole coffee beans because it's less messy and works great, but you can also use ground coffee if you don't have any whole beans.
Store the bottle out of direct sunlight, shaking the bottle occasionally, when you walk by it, to help incorporate all of the ingredients so they infuse well into the alcohol.
Depending on the strength of the alcohol used, the type of coffee, and whether it was ground or not, in just a few short days the rum will have taken on a dark color and a strong coffee flavor. You should also note the flavor of the vanilla beans, but the flavor is more subtle. At this point, it's really up to you how long you want to leave it infusing. When you are ready to stop the process, move onto the next step.
Make a sugar syrup. The first time I made this liqueur, I made a simple sugar syrup by mixing together the water and sugar in a pan over low to medium heat until the sugar dissolved. I used equal weights of water and sugar to make a very basic sugar syrup, but you can make it sweeter by adding more sugar, or lighter by adding less. Once cooled, add the syrup to the coffee infused rum.
I have since been experimenting with ways to alter the final flavor of the liqueur. If you want a more caramel flavor, you can caramelize the sugar first, before dissolving it in water to make the syrup. To do so, heat the sugar, without water added, to a pan over low to medium heat until it begins to melt and lightly brown. At that point, you can add water to dissolve the caramelized sugar into a caramel flavored sugar syrup.
Strain out the coffee and vanilla beans, and add the sugar syrup to the coffee and vanilla infused rum. It's a good idea to add the sugar syrup little by little, tasting the mixture along the way, so that you don't make the liqueur too sweet for your taste. Stop when you've reached the desired level of sweetness. (If it's sweet enough, but too strong, you can add a little water. I'd wait a few days before adding the water, though, because the alcohol flavor seems to slightly mellow with time.)
I wanted a stronger vanilla flavor, so I added a couple of vanilla beans back into the liqueur. Some people are very particular about not having little seeds in the final liqueur. If you want it perfectly smooth and clear of any seeds, you can do a final strain through a coffee filter or tight cheesecloth.
Store at room temperature or refrigerated, and serve alone or in any Kahlúa or Tía María recipe. (My husband adds a dash to his espresso.) Enjoy!