Preheat the oven to 350°F. Line an 8 x 8-inch pan with a parchment paper sling.
Cream butter and sugar. Add flour and salt. Press into the pan. Bake for 15-20 minutes until barely starting to brown around the edges. Remove from oven and decrease the oven temperature to 325°F.
3/4 cup salted butter, 1/3 cup granulated sugar, 1 1/2 cups (212g) all-purpose flour, Pinch of salt
Meanwhile, whisk sugar, eggs, egg yolks, and salt in a medium saucepan. Whisk in lemon juice.
1 1/2 cups (300g) granulated sugar, 4 large eggs, 2 egg yolks, Pinch of salt, 1 cup fresh lemon juice
Heat, stirring constantly, over medium-low heat until thickened and mixture reaches 160°F on a candy thermometer. Remove from heat and strain through a fine mesh sieve.
Add lemon zest and butter, stirring until butter is melted and combined.
1 Tablespoon lemon zest, 8 Tablespoons salted butter
Pour filling into the crust. Bake for 18-20 minutes until just set with a slight jiggle in the center. Cool completely and chill in the fridge for 2 hours or overnight.
Whisk egg whites, sugar, cream of tartar, and salt in a double boiler or large bowl that can sit over a small pan. Bring a few inches of water to a boil, then place a double boiler over water without letting it touch water. Reduce heat to a simmer.
4 egg whites, 1/2 cup (100g) granulated sugar, 1/4 teaspoon cream of tartar, Pinch of salt
Continue to whisk until sugar dissolves and mixture reaches 135-140°F on a candy thermometer.
Remove from heat and beat with a hand mixer until a billowy meringue with stiff peaks forms.
Spread meringue over chilled lemon bars. Use a kitchen torch to toast meringue swirls or broil for 30 seconds to 1 minute, watching carefully so the meringue does not burn. Slice into squares and serve chilled or at room temperature.