In the bowl of a stand mixer fitted with a paddle attachment, mix the flour, salt, cold butter, and powdered sugar until the mixture resembles coarse crumbs. Add the beaten egg and mix briefly, just until the dough comes together. Form the dough into a disk, wrap in plastic, and refrigerate for 30 minutes.

Remove the dough from the refrigerator and roll it out thinly on a lightly floured surface. Line a 9-inch/23 cm tart pan with the dough. You will have extra—wrap and freeze it for up to 2 months. Freeze the tart shell for 15 minutes.

Heat the oven to 350°F (180°C). Remove the tart shell from the freezer, line it with foil, and fill with pie weights or dried beans. Bake for 15 minutes. Remove the foil and weights, then bake for another 5 minutes, until the crust is lightly golden.

To make the filling, whisk together the eggs, sugar, lemon zest, lemon juice, and cream in a heatproof bowl. Place the bowl over a pot of simmering water, ensuring the bottom of the bowl does not touch the water. Whisk constantly until the mixture thickens and reaches 170–175°F (75–80°C).

Remove the bowl from the heat and whisk in the butter cubes until fully melted and incorporated. Let the filling cool to room temperature, then pour into the baked tart shell. Refrigerate until the filling is set, about 1 hour.

To make the meringue, whisk the egg whites and sugar together in a clean heatproof bowl. Place the bowl over a pot of simmering water and whisk constantly until the sugar is fully dissolved and the mixture is warm to the touch. Remove from the heat and whip with a hand or stand mixer until stiff, glossy peaks form.

Spread or pipe the meringue over the chilled tart. Toast the top with a kitchen torch or under a broiler just until golden. Serve chilled or at room temperature.