Heat a saucepan to low and add in the butter. Allow the butter to melt, then whisk in the monk fruit/allulose.
Whisk in the key lime juice, then the egg yolks, eggs, salt and finally the key lime zest.
Cook on medium-low for 6-8 minutes while whisking the entire time, until thickened enough to coat the back of a spoon.
Transfer to a bowl and cover with plastic wrap, making sure it's touching the curd. This will help prevent a skin from forming. Store in the fridge for up to a week in an airtight jar or freeze.
Makes about 1 ¼ cups.If you can't find key limes near you, you can substitute with regular limes and the curd will still turn out perfectly! Key Limes are slightly sweeter, so using regular limes will give you a slightly more tart curd.