Though some lifestyle gurus may claim to subsist on "chemical free" food, such a thing does not exist. Like the entire world around us, food is made up of complex chemical systems, and those systems can be manipulated to make olive oil powders, cocktail spheres, or cheese sauce from any cheese, even the ones that don't melt so well. If you know someone who views their kitchen as a laboratory, these gifts are for them.
A Sous Vide Immersion Circulator
Precise temperature control yields predictable, consistent results, and that is exactly was a sous vide system gives you. I love both the Anova Precsion Cooker and ChefSteps' Joule, though the latter requires the use of their (very sleek) app. Whichever machine you choose to give, know that you are giving the recipient the gift of perfectly cooked food with less stress.
A Vacuum Sealer
Though you can cook food sous vide in freezer bags, a vacuum sealing system makes it a bit easier. Not only does it help your bag-o-food stay submerged in the bath, but the thicker material won't break at higher temperatures. In addition to sous vide cooking, a vacuum system will keep all of your food fresher for longer, no matter how you cook it.
Any Book From Modernist Cuisine
Every single Modernist Cuisine book is a wealth of very specific answers to questions you didn't know you had, teaching one the science behind great cooking in great depth. Modernist Cuisine at Home is thorough enough for most home cooks but, if you've spotted a friend or family member checking out counter-top centrifuge systems, the original five-volume set might be for them. There's also a new, very extensive book dedicated to bread which - considering how much goes on at the molecular and bio-molecular level there - makes total sense.
A Kitchen Torch
Nothing looks quite as "mad scientist" as a blazing torch. Besides being a ton of fun to use, the kitchen torch can help you form crackling bruleed crusts, char some peppers, or add a bit or browned goodness to any food that could use a bit of flame. (You can also use it to torch marshmallows indoors, on a whim, which I think is very valuable.)
Fuel for Your Torch
Your torch will need fuel. Get this fuel.
An Immersion Blender
Proper emulsification is the key to truly excellent sauces and aioli, and an immersion blender is the best tool for the job. Silky soups, fluffy whipped cream, perfect mayonnaise, and heavenly hollandaise are all a cinch with this workhorse stick blender, and no kitchen - molecular or not - is complete without one.
iSi Whipping Siphon and Chargers
The molecular-obsessed are quite into foams - espuma if you're truly fancy - but these pressurised canisters can also be used to infuse flavour into booze, quick pickle vegetables, and carbonate pieces of fruit.
Not all cheeses like to melt. If you're a grilled cheese enthusiast, you've no doubt noticed a difference between the smooth, evenly-melting American and a harder cheese like cheddar, which can separate into an oily mess if overheated. Melting salts prevent this tragic separation by acting as an emulsifier, increasing the pH of the cheese and making the proteins more soluble. Simply put: These salts can make any cheese melt like American, or even Nacho, cheese - a feat I think should be classified as a "Christmas miracle".
There are pizza stones, and then there is the Baking Steel. This steel sheet is super conductive, which means crispier, more evenly cooked pizza crusts both in the oven and on the grill. It can also help you roast a turkey faster and, when popped in the freezer, can be used as a cold plate or anti-griddle (which means you can make your own ice cream rolls).
Stuff for Making Fancy Food Pearls
Spherification is the precious process of taking any flavorful liquid and giving it the appearance and texture of caviar. It can be done with booze, juice, broth, syrups or any flavorful liquid, as long as it has a pH of 3.6 or greater. All you need is a syringe, plus to the two chemical compounds below, and a sense of culinary whimsy.
Maltodextrin for Making Tasty Powders
If you have a friend or a loved one just dying to make powders out of Nutella, bacon fat or olive oil, you need to get that person some maltodextrin. This stuff can turn any high-fat liquid into a powder for culinary dusting purposes and - at only a few bucks - it makes a great stocking stuffer.
A pH Meter
When getting truly molecular, there are times - such as during the above spherification process - when one needs to know what pH they're working with. There is no way to eyeball this, so a pH meter is needed. Beyond fancy molecular techniques, it can also keep you safe from botulism when making fermented garlic honey, which is something you should definitely be making.
A Modernist Cooking Starter Kit
If you'd rather give a kit than a bunch of little bags of chemical compounds, consider this one from Modernist Pantry. Not only does it have all the little bags your budding gastronomist will need to make spheres, powders and foams, but it also comes with a spherification set up, a scale, and a book full of recipes and techniques.