Measure allulose into a mini food processor, let it run until the allulose is powdered. Leave the lid on - letting the dust settle - while you start with the egg whites:

In clean stand mixer - or with an electric hand mixer - whip egg whites until foamy.

Add lemon juice, whip for few seconds to combine.

Slowly add powdered allulose until cookie icing reaches desired consistency. You will want a fairly thick frosting – but still smooth and workable – for piping details and borders.

If you're not using it immediately, transfer to an airtight container, cover surface with plastic wrap, and secure lid tightly.

Re-whip before use.