Rub sugar and zest together in a metal bowl until well incorporated and fragrant
Let sit for 30 minutes
Add eggs and whisk to combine
Create a double boiler over the stove by setting a bowl over a pot with 2 inches of boiling water
Fit bowl with lemons, sugar and eggs over the pot of boiling water
Heat sugar, lemon and eggs for 5 minutes, stirring constantly
Whisk in lemon juice and 4 tablespoons of butter
Whisk constantly until mixture reaches Fahrenheit 180°, or becomes a custard like consistency
Swap out whisk for hand beaters and begin beating the mixture over the heat while adding the butter
Add butter one tablespoon at a time until incorporated
Remove from heat and let cool to room temp stirring occasionally while cooling
Cover with a sheet of plastic wrap to prevent a crust from forming, and refrigerate to chill
When set, whip cream to stiff peaks with two tablespoons white sugar
Fold cream into lemon curd
Spoon lemon cream into tart crust shell and serve