Here's What You Can Substitute For Bitters
If you've ever worked in a bar or restaurant, you're probably accustomed to using bitters. If not, well, it's totally possible you have never even heard of them, even if you have consumed them before. According to VinePair, bitters are the name for a group of liquor-based flavorings used in cocktails and drinks that are generally sold in small bottles. Recipes normally only require a few drops of these strongly flavored liquids to acquire the desired taste for your drink.
Bitters are made by infusing a neutral spirit with aromatics. These aromatics can include seeds, spices, barks, roots, or fruits, among other ingredients. They were originally thought to have health benefits, but eventually mostly came to be used as flavoring agents for cocktails and low-alcohol drinks. The most commonly called for bitters in recipes are Angostura bitters and Peychaud's bitters, though smaller brands make a vast assortment of different types of bitters which can be used to add a variety of flavors and complexity to your drink.
You cannot easily replace bitters in a recipe
While bitters are becoming more and more universally available, especially Angostura and Peychaud's bitters, it can still sometimes be tricky to find them if you are in a rush. Culinary Lore has bad news if you are trying to find a true replacement for bitters in your recipe without suffering any loss of complexity and flavor. They state that there is simply no substitute for bitters that can be commonly found in the household, though you can try using a citrus peel (making sure to not include any of the flesh) to replicate some of the taste that bitters would impart.
While you can make your own bitters at home, the process is time consuming and you are more likely to find them at a store or have them shipped from an online retailer before your homemade ones are ready to consume.
Above all, the best replacement for any bitters in a recipe is another type of bitters. If you are unable to get your hands on any bitters at all, Bitters Club offers one other option for replacing Angostura bitters specifically. They state that you can make a flash infusion by grinding whole spices with a mortar and pestle and letting the spices sit in one to two tablespoons of vodka or rum for five minutes. Strain the solids out and you might have something to work with.