Alcohol-optional desserts?

The only dessert I can think of where alcohol would be truly done on a portion by portion basis would be the type of ice cream parfait we used to serve at a restaurant I worked at in my long ago youth: it was essentially vanilla ice cream layered in a parfait glass with a sweet liqueur such has au de noisette or Framboise--the possibilities are endless. You will note this is essentially a minimalist adult sundae, so you could easily have one made with plain caramel sauce or chocolate sauce for the designated driver.

Still, if the issue is a designated driver, rather than a recovering alcoholic who can have no alcohol whatsoever, I wouldn't worry overmuch. Many desserts which contain alcohol have very little per serving, and should not be an issue for the driver. Some of these are spectacular show pieces. Some you might consider are:

• Bananas Foster
• Crepes Suzette
• Rum cake
• Truffles (as lemontwist mentioned in their comment answer)
• Trifle

Another option would be any number of rich coffee based drinks, where you could omit the liquor for the designated driver's portion. Not truly a dessert, but in line with your theme.

Lastly, I don't think a party of any sort, even a cocktail party, requires desserts with alcohol in them. Be free. Serve what you like. Serve no dessert at all if you like.

Crepe suzette?

Then crepes and ice cream, or chocolate sauce, is good for the drivers. As a side note, the amount of alcohol in most deserts would be well below the limit for driving in most regions.

Flambeed crepes is one option (grand marnier (for an orange taste), calvaldos (with apples), cognac, or similar).

Creme brulée can also be very spectacular, if you already have a hard sugar crust, you can use something quite high proof, like everclear (flavorless) and extra sugar, and then set it on fire right in front of your guests. I recommend practicing first so you can get maximum effect.