Heat your oven to 350F [180C]. Line an 8-by-8-inch [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. Place the graham crackers, sugar, and salt in the bowl of a food processor and process until the fine crumbs form. Add the cold butter and pulse until the cold butter blends into the crumbs. 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.

Make the filling: [See Note up top for using different lemons.] Trim the stem end of the whole lemon and cut it into thin slices. Remove any seeds. Add to a food processor or blender jar (preferably a high speed blender) along with the lemon juice, egg yolks, butter, sugar, vanilla (if using), and salt and blend until very smooth, scraping down the sides as needed. 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, and might even look like a total mess (unevenly browned or bubbly), but it will set up as it cools. Place on a cooling rack. 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.

Make the meringue: An hour or so 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, or the back of a spoon or offset spatula to make swirls.

Finish the bars: When you’re ready to finish, use a kitchen torch or your oven’s broiler to brown the meringue. Refrigerate until ready to serve. Use a knife dipped into hot water to cut bars into 12 to 16 squares, depending on how large you want them.