Preheat the oven to 325°F. In a saucepan, melt ½ cup allulose with the water over medium heat until deep amber, then quickly pour into 6 ramekins, tilting to coat the bottoms.
Whisk the eggs, remaining ¼ cup allulose, cream, milk, vanilla, and salt until smooth.
Strain the custard and pour over the caramel in each ramekin.
Place the ramekins in a water bath and bake 40–45 minutes, until just set with a slight jiggle in the center.
Cool, then chill at least 4 hours.
Run a knife around the edge and invert onto a plate to serve.