To make the lemon curd:

In a medium heatproof mixing bowl, place eggs, sugar, lemon zest, lemon juice, and heavy cream, if using, and whisk to combine. Place the bowl over a saucepan of simmering water (bain-marie). Cook on moderate heat, whisking constantly, until mixture becomes thick (mine was ready in 10 minutes, but it can take up to 20). If you have a thermometer, it should register 170°F/75°C; otherwise, it should coat the back of a wooden spoon and leave a clear pass if you run your finger through it. The curd will thicken more once cooled.

Remove from heat and immediately strain mixture through a fine mesh sieve. Add butter, a few cubes at a time, and whisk until completely melted and incorporated, and mixture is smooth. Take your time with it—the whisking makes for an airy and light texture. Allow to cool to room temperature before filling the tart. (Lemon curd can be kept in an airtight container in the fridge for up to a week, or can be frozen for up to 2 months. To thaw, place overnight in the fridge. Whisk the mixture to smoothen it before using.)