Some recipes call for adding butter once the curd is thickened and the pan is off the stove. Upon testing I found it results in a grainy texture. Add butter when the pan is still on the stove and the curd is coming together, just before it thickens. This will give you smooth and velvety curd. Remember to cube butter before you add it to the pan. Helps melt butter better and fast without ending in overcooking eggs.