• Preheat the oven to 350°F (175°C). Line an 8x8 or 9x9-inch square pan with parchment paper, leaving some overhang on the sides for easy removal if you want to make bars. Or if you want to make a pie, use a pie shell or tart shell.

Graham Cracker Crust

• In a food processor add graham cracker crumbs, sugar, salt, and melted butter. Pulse until the mixture resembles wet sand. Press firmly into the bottom of the prepared pan to form an even layer.
• Bake for 10 minutes. Let it cool while you prepare the filling.

Lemon Pie Filling

• In a large bowl, whisk together the sweetened condensed milk and egg yolks until smooth. Add the lemon juice and zest, and whisk again until fully combined. The mixture will thicken slightly on its own.
• Pour the filling over the warm crust and smooth the top. Bake for 15–18 minutes, or until the edges are set and the center looks just slightly jiggly.
• Let the bars cool, then refrigerate for at least 4 hours (preferably overnight) until fully set.
• Use the parchment overhang to lift the bars out of the pan. Slice into squares or rectangles. Wipe your knife clean between cuts for neat edges.

Meringue Topping

• Fill a small pot with 2 inches of water. Place it over medium heat. Bring to a gentle simmer.
• Add the egg whites, sugar, cream of tartar, and salt to a large glass bowl, and whisk to combine.
• Place the glass bowl on top of the pan to form a double boiler.
• Continue to whisk the mixture over the double boiler, until it reaches 140ºF, or until the mixture is just barely hot, and the sugar has entirely melted.
• Remove from the double boiler. Either whip the mixture in the bowl it already is using a hand mixer, or transfer the syrup to the bowl of a stand mixer, and whip using the whisk attachment.
• You should whip the syrup for about 10 to 15 minutes, or until stiff peaks form. The mixture should be thick, elastic, glossy, forming stiff peaks.
• Add the vanilla and mix to combine.
• Spread over the bars, toast with a torch if desired.