Start with room temperature eggs. This helps prevent curdling. I had no curdling at all as my eggs were at room temperature for a few hours.

Place a stainless steel mixing bowl over a pot of simmering water, without the bottom touching.

Whisk together eggs, lemon juice, zest, sugar, over simmering water,

Continue to whisk for about 10 minutes or until a wooden spoon is coated, or until you reach 170 degrees.

Strain through a fine mesh strainer. Whisk in butter. Add heavy cream, whisking to thoroughly incorporate.

Place plastic wrap touching curd and chill. Cool completely before using, unless using for baked lemon tart.