Set a glass or ceramic bowl over a pan of barely simmering water.
Whisk together the eggs, sweeteners, lemon juice, and lemon zest. Whisk continuously until the mixture thickens, about 10 minutes. Watch carefully as it will thicken all of a sudden.
Immediately remove from heat and add the butter. Let sit a few minutes to melt and then whisk until smooth and creamy.
Refrigerate 2 to 3 hours to firm up.
Storage Information: Store the curd in a glass jar in the fridge for up to a week.Sweetener Options: I find that a mix of erythritol and allulose or BochaSweet produces the best results. It firms up nicely but doesn't recrystallize. You can use whatever you like best but know that the consistency may change. Use a powdered sweetener if using erythritol, to reduce grittiness.