Step 1

Preheat the oven to 350F [180C]. Line an 8 by 8 in [20 by 20 cm] baking pan with two pieces of parchment trimmed to fit, going in both directions, with some extra hanging over for easy removal of the bars later.

Step 2

Place the graham crackers, sugar, and salt in the bowl of a food processor and process until the fine crumbs form. Add the melted butter and pulse until well blended. It should look and feel like wet sand. Transfer to the prepared pan and mix it up with your hands  to make sure the butter is well distributed. Press into the pan, going up the sides a bit, and bake for 10 minutes, or until just golden. Let cool while you make the filling.

Step 3

Trim the stem end of the whole lemon and cut it into 8 pieces. Remove the seeds. Add to a blender jar (preferably a high speed blender) along with the lemon juice, egg yolks, butter, sugar, vanilla, and salt and blend until very smooth. Pour over the crust (it’s ok if it’s still warm) and bake for 30 minutes, or until it is bubbling and browning around the edges. It won’t look at all set, but it will set up as it cools. Place on a cooling rack.

Step 4

After about 10 minutes, run a small, sharp knife around the edges. Cool completely, then chill until cold. When completely chilled, carefully remove the parchment and, using a spatula, transfer to a small baking sheet (you can do this just before adding the topping).

Step 5

A few hours before serving, make the topping. Combine the egg whites, sugar, vanilla, and salt in the metal bowl of a stand mixer and set over a pan of simmering water. Keep the mixture moving, using a whisk or the whisk attachment, until the sugar is completely melted and it’s hot to the touch (or 160F). Transfer to a stand mixer and beat on high speed until glossy and very stiff, 2 to 3 minutes. Transfer to the top of the lemon bars, smooth out, and use a large serving fork to create a pattern in the meringue.

Step 6

When you’re ready to finish, use a kitchen torch to brown the meringue (you can use a broiler, too). Refrigerate until ready to serve. Cut into 12 to 16 squares, depending on how large you want them.