Cook the allulose and water in a clean dry saucepan over medium heat, without stirring, until light amber, 5 to 8 minutes. Swirl to even the colour, then pour into a cake pan or ramekins, tilting to coat the bottom.

Warm the cream over low heat until steaming, not boiling. Whisk the eggs, yolks, sweetener, vanilla, and salt smooth, then slowly whisk in the hot cream. Strain through a fine sieve.

Heat the oven to 325°F. Set the mold on a folded towel in a roasting pan, pour in the custard, and add hot water halfway up the sides. Cover loosely with foil.

30 to 40 minutes for ramekins, 45 to 60 for a larger pan, until the edges are set and the centre jiggles like soft gelatin.

Cool, then refrigerate at least 4 hours. Run a knife around the edge and invert to serve.