Add the egg yolks, lemon juice, and sweetener to a saucepan and whisk until fully combined.

Place the pan over medium-low heat and whisk constantly to prevent sticking or curdling.

Once the mixture reaches a gentle boil, reduce the heat to low and continue whisking until the curd thickens and lightly coats the wires of the whisk. This takes only a few minutes.

Remove the saucepan from heat and immediately whisk in the butter until smooth.

Allow the lemon curd to cool slightly, then transfer to a mason jar or airtight container and refrigerate. Consume within one week.