I have made a million of these in my time, and also learnt the tricks for customers who didn't want sugar etc. so there are always ways to make it work! Please never use whipped cream though - it ruins them!
Its simply a matter of science. The cream floats because of the solution of coffee, sugar and alcohol. The sugar is important as it supports the cream.
You should use tall glass latte mugs, or if you cant get hold of these (then wine glasses are suitable)
Buy Irish Whisky and make good filter coffee.
This method is guaranteed.
Put a metal spoon in the glass and fill it with hot water. Allow to stand so that the glass is hot, now pour away the water.
Now add a generous scoop of brown sugar to the glass and add a generous 30ml of whisky. Pop the metal spoon back in and pour coffee into the glass so that there is a good1/2 inch left at the top.
Stir the coffee to make the sugar melt, once you can see that there are no granules floating around, it is ready to add the cream.
Use double cream, poured from the carton into a jug to make it easier for you. Stir so that there are no lumps!
Take the spoon that was in the coffee, shake it dry and place spoon side up (not upside down) so that it is just touching the coffee and the side of the glass. Tilt it towards the glass side.
Slowly pour cream into the spoon. Do not move the spoon as the coffee fills it and starts to flow around the sides of the glass.
Once the top of the coffee is covered gently remove the spoon.
If guests dont want sugar you can still do it this way, just give the cream a good shake to get a little air in (dont whip) and be more careful when pouring. With no sugar coffees the cream does float initially bit will start to mix with the coffee really quickly. You need to get these out to the table as soon as you pour them.
You can use less alcohol if you wish, or a different alcohol to make different types of coffee.
Never sprinkle chocolate on the top - at the most you could float a coffee bean on top of the cream! Mashed up flake goes well on top of the cream if you make a coffee without any alcohol - otherwise the taste of the chocolate taints the alcohol.