Floating
Floating is a technique that can help you achieve not only a stunning presentation but also a burst of taste that evolves as a drink is consumed. This is achieved by delicately placing an ingredient atop the drink's surface, creating a distinct visual and flavor layer. Floating is a minor technique, used in 2.5% of the recipes in Tipply, and is applied to a subset of ingredients within a recipe. These recipes include the Dark and Stormy, New York Sour, and Puka Punch.
Bitters and infused oils may also be floated in a cocktail, typically as droplets on the drink’s surface and not as a distinct layer.
Why float
It is beneficial to float an ingredient in a mixed drink in two scenarios. First and the most obvious is the aesthetic reason. The visual distinction of layers adds interest to a drink. Second, is the ability to use a floated ingredient to evolve the taste of a drink over time. It is common in tiki recipes to float an overproof rum - combined with a straw. The point isn’t to slurp the potent rum immediately. Instead, as the drink is consumed through the straw, from the bottom first, the drink also becomes more diluted as the ice melts. By the time the overproof rum enters the straw and is tasted, it will have diluted to an agreeable level and maintain a steady ABV throughout the life of the drink. Instead of a hammer to the face, the drink received a loving slap.
What you’ll need
There are many ways to float an ingredient, from using a 
When comparing barspoons for floating, I find a more narrow spoon bowl to be useful, as they fit more easily into a wider array of glassware (i.e., flutes and fizz glasses). Of course, I’m a sucker for the precision of pipettes, but those are just one more item to use, are fragile, more finicky to clean.
How to float
My first attempts at floating were frustrating, resulting in indistinct layers and a sloppy looking presentation. Here's the technique that works for me. Take a barspoon and place at an angle inside the serving vessel, with the bowl inverted. Move the tip of the barspoon so that it is just below the surface of the drink and touching the edge of the vessel. Now, slowly pour the float ingredient across the back of the spoon's bowl so that the ingredient flows down the spoon then down the edge of the glass. The barspoon and glass's edge will slow the velocity of the float ingredient encouraging it to stay on the surface.
If you’re intending to float drops of bitters, oil, or similar, the technique is even easier. Just use a dropper and add the needed amount artistically to the surface of the drink. You’ll get better results the closer your dropper is to the surface. Drop, move, drop, move. Simple.
Be mindful that a heavier ingredient will be more difficult to float. What’s a heavier ingredient? Think sweeter, and/or lower ABV ingredients. These all have a higher specific gravity, making them denser or heavier. Rum has a specific gravity of 0.94. You’ll have a frustrating time attempting to float maple syrup, which has a specific gravity of 1.39, on top of the rum. Let’s consider the White Russian, AKA the Caucasian. It’s easy enough to build the drink by pouring the vodka, espresso liqueur, and cream together into a glass and swirling with your Lebowski finger. My preference is to combine the vodka and espresso liqueur, then to float the cream on top. The slightly denser cream won’t produce a distinct layer, but instead will sink slowly into the rest of the drink, producing a visually appealing bit of turbidity. The drinker can stir their beverage after receiving and admiring it. Now if you instead were to use whipped cream, which is lighter and fluffier because of the foam matrix, you’ll never get that to sink.
Another thing to keep in mind is that you may need to give some advice to the person drinking your concoction. Maybe people have this trained aversion to straws, reusable, biodegradable, or not. Often, they’ll remove the straw from their drink, thinking it doesn’t serve a useful purpose. But if you’ve floated a lovely 2 ounces of Lion Hart 151 5um on top, they’ll be in for an unwelcome surprise. Instead, guide them to use the straw, so that overproof rum is deliciously diluted at the tailend of the drink.
Floating isn’t a difficult technique, it just takes a steady hand and some forethought into the tendency of which ingredient naturally wants to be on top. Just remember to pour slowly and use an implement such as a barspoon and the glass’s edge to further slow the float ingredient. Reducing the distance the liquid has to travel, as well as the force behind it, will produce better results.