Traditionally flavoured with Amaretto liquor (hence the name) I have used sugar-free almond extract instead. These chewy biscuits are almost keto naturally and really only require the substitution of a quality low-carb sweetener for the sugar.
You can use packaged ground almonds or grind your own; if skin is retained your amaretti will be darker in colour but still flavoursome. If you grind your own, make sure to toast the almonds first, as it will intensify the flavour and make your amaretti even more delicious.
Arguably these keto amaretti work best with erythritol sweeteners (think Lakanto and powdered Swerve ). Xylitol works best taste-wise, but it takes ( much) longer to ‘dry out’ so your amaretti will be extra delicate for about a day and the edges won’t crisp up much.
Then you need an egg white, almonds, amaretto liqueur, almond extract and sweetener. Actually, you don’t ‘need’ amaretto liqueur, but it does impart a stronger flavour to the biscuits, so I recommend it. I know it’s expensive, especially if all you can get is the well known brand Amaretto di Saronno.