The Bloody Mary already works very well for halloween. But to push up the scare factor, we'd recommend thinking about the garnishes and floating a couple of radish eyeballs is certainly the way to go.
The Black Russian is another classic that lends itself well to halloween, given its dark colour. To make this one more halloweeny we'd change the glassware to something more skull like perhaps and use a bit of dry ice.
We don't keep it a secret that Margarita is our personal favourite at Jiggr. So how could we not include it in halloween? The best way we feel to halloween-ify a classic margarita is to add a splash of red wine or sangria after the margarita has been mixed and added to the glass. This actually works best when you make a frozen margarita as the red colour stays on the top and starts to bleed through the drink nicely.
The espresso martini is a party pleaser and given its classic look there's not much more you need to do to this one to make it more scary. But this is where it get's a bit weird, for this one we'd recommend adding REAL edible bugs, instead of the classic coffee beans. Edible bugs are easily found online and will certainly make this one a terrifying experience.
Our second martini suggestion, the Black Martini. T?his one is another good candidate for some eyeballs, but this time we'd use lychee, but a few squashed blackberries will add to the effect.
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