Add the sweetener and water to a small heavy-bottomed saucepan over medium heat. Stir until the sweetener has dissolved and the liquid looks clear, 2-3 minutes.

Add the butter. Let it melt and simmer, stirring now and then, until the mixture bubbles and turns golden amber at the edges, 4-6 minutes. Watch it closely, as allulose browns faster than sugar; turn the heat down to medium-low if it darkens too quickly.

Take the pan off the heat and stir in the baking soda. The sauce will foam up, then settle. Stir in the apple flavoring, then taste and add another drop or two if you'd like a stronger apple flavor.

Sprinkle the xanthan gum slowly over the surface while whisking constantly, and keep whisking for about 30 seconds until smooth. The sauce thickens more as it cools, so let it stand for a minute before deciding whether to add more thickener.

Serve warm, drizzled over pancakes, waffles, ice cream or cheesecake. Refrigerate leftovers in a sealed jar and rewarm gently before serving.