Once the curd is very thick, remove from heat and strain through a fine mesh strainer into a small bowl. Let mixture cool about 20 minutes or until an instant-read thermometer registers 140°F. Stir in the butter and, if using, the vanilla paste or vanilla. If using vanilla bean, using the tip of a paring knife, slit vanilla bean lengthwise down the center. Using the knife, scrape the seeds from the vanilla bean into the lemon curd.