Preheat oven to 400ºF. Roll out the puff pastry to fit your pie pan. Prick the bottom several times with a fork. Cover with a piece of parchment paper (or aluminum foil) and fill it up with pie weights (or you can use dry rice or other grain, or dried beans - you just won't be able to cook with them after). Bake the pie crust for about 20 minutes, until nicely golden brown.

Meanwhile, whisk lemon juice, zest, sugar and salt together in a medium saucepan over medium heat. Cook until steaming hot and sugar is dissolved.

Whisk eggs in a heatproof spouted measuring cup. Temper the eggs (prevent curdling) by very slowly drizzling about 1/2 cup of the hot lemon mixture into the eggs while whisking constantly. Again pouring very slowly,  whisk the egg mixture into the saucepan with the rest of the lemon mixture. Cook the lemon curd, stirring constantly, until it thickens enough to coat the back of a spoon (if you're unsure, pull out your thermometer - it's done at 170ºF). It shouldn't come to a boil. Whisk in the butter, then scrape into baked crust (and if it's curdled at all, don't sweat it, just force it through a fine-meshed sieve first to remove any scrambled bits).

Refrigerate 1 hour or until filling is set, then top with whipped cream and serve.