Step 1

Combine all ingredients except butter in double boiler over simmering pot of water. Heat, whisking constantly, for 10 minutes until the mixture has thickened enough to coat the back of a spoon (betweeen 160-170 degrees F on a candy thermometer).

Step 2

Remove from the heat, then add the butter, whisking until combined.

Step 3

Pour into a bowl and cover with plastic wrap pressed right onto the curd. Refrigerate until cold.