Whisk the cream with a teaspoon or two of sugar just until thickened but it doesn't hold peaks.
Warm a cup and add the baileys, rum, and sugar.
Pour in the hot coffee and mix well.
Carefully spoon two tablespoons of the cream over the top and dust with cinnamon or sugar crystals. See notes.
Notes
Expert Tip: You can make a non-alcoholic version of this by using a tablespoon Irish Cream coffee syrup, ½ tablespoon cinnamon coffee syrup, and a little rum extract.
Pour boiling water in the coffee mug to warm it up before adding the other ingredients. Just pour the water out and add your coffee - it will stay hot a lot longer.
In order for the cream to float on top properly it needs to be pourable and not too thick... but if it's not thick enough it will break through and sink into the coffee.
The easiest way to get the (properly thickened) cream to float is to turn a spoon upside down and place the tip of the spoon against the coffee cup just over the coffee. Slowly pour the whipped cream over the spoon allowing it to gently cover the top of the coffee.