A French press
Bring Out Your French Press To Make Better Cocktails
A French press’ ability to soak and separate grinds in hot water and create a strong brew without muddying the mug makes it a perfect tool to infuse solid ingredients in cocktails.

The device’s "pressing part," which requires pushing its plunger down, creates pressure that aerates concoctions, allowing for a deeper infusion while leaving your drink pulp-free.

Alcohol is already a highly effective solvent, so the additional pressure will help transport the flavors of fruit, herbs, and other aromatics like few other methods can.
However, since cocktails are mostly served chilled, the ingredients will need to sit a bit longer in the device than coffee or tea, with some drinks needing 15 minutes to process.
It’ll help to muddle the aromatics before adding them to the French press to extract the maximum flavor. The filter in the device will separate the little bits once you plunge it.
Remember that the lower the alcohol content, the more time it’ll take for the infusion. Be sure to use a shaker to add additional mixers once you've already pressed down the drink.
In this case, the main purpose of the French press is to infuse the flavor notes of solid ingredients into alcohol, essentially helping create homemade bitters.