Think of bitters like the spice rack of the cocktail world. That's the first thing Hunter Bryant, bar manager at Haven in Tampa, tells me when I sidle up to the restaurant's bar on a recent weekday for a Bitters 101 lesson. It instantly helps me understand the allure of the alcoholic extracts. Like most people, I was familiar with bitters, those mysterious dark dashes of potent liquid gingerly administered to a cocktail, but never too sure what function they served. I've seen Angostura and Peychaud's, two popular national bitters brands, in the liquor store, always intimated to bring them home. Up until a couple of years ago, I only regularly encountered them in something like an Old Fashioned, a classic drink made up of a spirit (usually bourbon or whiskey), water, sugar and bitters. But sitting at Haven's bar with dozens of bottled brands and some of Bryant's own concoctions in front me, I begin to see why bitters may be more popular than ever, and a crucial part of the recent craft cocktail explosion. "The most basic definition of bitters is a strongly concentrated infusion of plant material: bark, roots, flowers, fruits and other accentuating flavors," Bryant says. "They're used in very small amounts, drops and dashes, to add secondary flavors to cocktails and tie all your other flavors together. A balancing element, just like salt and pepper."And bitters go way back. Bryant says they were an important part of what we now consider the modern cocktail, created a couple hundred years ago. Bitters were used to flavor spirits that were not very high in quality or flavor, combined with things like fruit to make drinks more palatable for those ancient imbibers. And because of their relatively low alcohol content (usually around 30 or 40 percent), they were also actively sold throughout Prohibition, considered a "nonpotable" ingredient as opposed to an alcoholic beverage. Bitters start with a high-proof alcohol, something like Everclear or a high-proof whiskey or rum. The higher the proof, the more efficient the extraction of flavor will be. What comes next is up to you. The classic infusions include esoteric-sounding ingredients gentian root, bitter orange peel, cinnamon chips, galangal and cinchona bark. These very bitter plant materials are the main components in something like Angostura. But I quickly learn that you can turn just about anything into a small bottle of potent bitters. Bryant's own collection, which he creates for fun but not yet officially as part of Haven's bar program, includes a large variety of infusion ingredients: black walnut, strawberry, banana, vanilla bean, hot pepper, dill, lime, grilled pineapple and blood orange. Story Stuart, who opened St. Petersburg's Story Brooke Craft Coffee Bar in February, even uses bitters in coffee. Before opening the shop, Stuart was a food and beverage experience manager at Walt Disney World, and she brings a lot of that cocktail knowledge and experimental spirit to her coffee menu. One of her signature drinks, called the Campfire Story, blends sweetened condensed milk with house-made vanilla syrup and chocolate bitters (as well as a bit of smoke and toasted marshmallow garnish for maximum campfire vibes). "When I was opening the shop, I knew I wanted to mix bitters with coffee; they just go so well," Stuart says. "It adds a different depth and complexity." Currently, she's working on a batch of chai bitters, brewing a concoction of spices and alcohol in a large glass jar that she keeps behind the counter. When I stop in one morning to learn more, she opens the lid and lets me smell it. It smells like Christmas — cinnamon and other warm spices — with a potent overtone of alcohol. She mimics the puckering face she made after a recent tasting of the unfinished batch. Stuart switches out her house-made bitters depending on what kind of drinks she's serving at the moment. These chai bitters will eventually go into a play on dirty chai tea. "Bitters started off more medicinal, you know, creating tinctures, using different herbs and roots," she says. "But now it's really interesting to taste the flavors of bitters and see how they interact with different things. Really you can take it anywhere, which is why it's so interesting."Bitters are also increasingly a key component in restaurants and bars that don't sell traditional spirits. Places like the Reading Room in St. Petersburg, which opened earlier this year with a lineup of nonliquor cocktails, mix Tiki Bitters, Peychaud's and more with things like prosecco and vermouth to create cocktails that do not skimp on flavor.Back at Haven, Bryant opens the small mason jars full of his bitters creations, using an eye dropper to place a couple of drops into my hands and his. He slaps his palms together, rubs them for a few seconds, waves them in the air a couple of times, then holds them up to his face and takes a big whiff. I do the same thing, and pretty soon we're both inhaling the scent of black walnut, rhubarb, vanilla bean. It's a surprisingly effective way to get the gist of each batch.He asks me to pick out three flavors, and adds a couple of drops of each to what looks like a glass beaker. After some stirring, we do the palm-smelling test again, and this time the flavors have merged to create an entirely new blend. Bryant calls it Blackberry Cobbler. This is often how it goes with bitters: a little of this, a little of that, and soon you've got something special."There's really no rules, the sky's kind of the limit," Bryant says. "As long as the craft cocktail movement stays popular, I think we'll continue to see a huge resurgence in bitters as well. They go so hand in hand, and they're so crucial to each other."Contact Michelle Stark at [email protected] Follow @mstark17.