Preheat your oven to 325°F (165°C).
Heat the allulose in a small non-stick pan over medium heat, stirring constantly until it melts and turns into a deep golden amber liquid.
Quickly pour the melted allulose into the bottom of a 6-ounce ceramic ramekin, swirling it to coat the entire base before it hardens.
In a blender, combine the whole eggs, egg whites, vanilla whey protein, almond milk, and vanilla extract.
Blend on a low setting until the mixture is completely smooth and incorporated, avoiding excessive foam or air bubbles.
Pour the custard mixture over the hardened caramel in the prepared ramekin.
Place the ramekin into a deep baking dish and fill the dish with hot water until the water level reaches halfway up the side of the ramekin.
Bake for 35 to 40 minutes, or until the edges are set but the center still has a slight, gentle jiggle.
Remove the ramekin from the water bath and allow it to cool to room temperature on a wire rack.
Refrigerate the flan for at least 4 hours, or ideally overnight, to allow the texture to fully set and the caramel to liquefy.
To serve, run a thin knife around the inside edge of the ramekin and invert the flan onto a small plate, letting the amber sauce flow over the top.