Toast the pecans: Place chopped pecans in a dry skillet over medium heat. Stir occasionally for 3 to 5 minutes until fragrant and lightly browned. Transfer to a plate to cool.

Melt and brown the butter: In a medium, heavy-bottomed saucepan, melt the butter over medium heat. Let it foam and cook until it turns a light amber and smells nutty, 3 to 4 minutes. Watch closely to avoid burning.

Add the sweetener: Stir in allulose (or allulose blend). Cook, stirring frequently, until it dissolves and the mixture looks glossy and syrupy, about 2 minutes.

Pour in the cream: Lower the heat slightly and carefully add the heavy cream. The mixture will bubble vigorously. Keep stirring until smooth.

Simmer to thicken: Let the sauce gently simmer for 3 to 6 minutes, stirring often, until it coats the back of a spoon. The sauce will thicken more as it cools.

Flavor it: Remove from heat. Stir in vanilla and sea salt. Taste and adjust salt or sweetness as needed.

Add pecans: Stir in the toasted pecans. If the sauce seems too thick, whisk in 1 tablespoon of water at a time until it reaches the texture you like.

Optional thickening: If you want a thicker sauce, sprinkle in a tiny pinch (up to 1/8 teaspoon) of xanthan gum while whisking vigorously to avoid clumps. Start small; it thickens quickly.

Serve warm: Use right away over keto ice cream, pancakes, waffles, mug cakes, or cheesecake. Or let it cool and store for later.