• Preheat your oven to 325°F. Grease an 8-inch square baking pan, line with criss-crossed pieces of parchment paper that overhang the sides, and grease the parchment paper. Set the pan aside.

Make the crust.

• In a medium-sized bowl, combine 1 cup (140 g) of the flour, confectioners’ sugar, salt and lemon zest and whisk to combine, breaking up any clumps of lemon zest.
• Add the butter and mix with a fork until well-combined. Press the mixture into the bottom of the prepared baking dish in an even layer.
• Place the baking dish in the center of the preheated oven and bake for about 15 minutes or until firm.
• Remove from the oven and allow to cool briefly.

Make the custard layer.

• In a medium-sized bowl, place the eggs, granulated sugar, baking powder, lemon juice and remaining 2/3 cup (93 g) flour, whisking vigorously to combine after each addition. Pour the custard mixture into the baked crust.

Bake the bars.

• Return the pan to the center of the oven. Bake until just set (about 25 minutes).
• The custard is set when it does not jiggle more than a tiny bit in the center when the pan is shaken gently back and forth.
• Remove the pan from the oven and allow to cool in the pan for about 20 minutes.

Chill the bars.

• Place in the refrigerator to chill until firm, about 2 hours and up to overnight.
• Remove the bars from the pan by running a butter knife or thin spatula around the perimeter of the baking dish, and then lifting the bars out of the pan by the overhung pieces of parchment paper.
• Dust lightly with confectioners sugar, and slice into 9 or 12 squares with a large knife. Serve chilled.