• 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 blend, confectioners’ sugar, salt and lemon zest and whisk to combine, breaking up any clumps of lemon zest.
• Add the melted butter and mix with a fork until well-combined. The mixture will be clumpy.
• Scatter the clumps evenly across the bottom of the prepared baking dish. Then use a small offset spatula or knife to press the mixture into a smooth, even layer on the bottom of the dish.
• Place the baking dish in the center of the preheated oven and bake for about 15 minutes or until no longer shiny. When it's done, it will have a matte look to it and be very pale in color.
• Allow the crust to cool briefly while you make the filling.

Make the filling

• In a separate large mixing bowl, place the remaining 2/3 cup (93 g) flour blend, granulated sugar, and baking powder, and whisk to combine.
• Add the lemon juice and the eggs, and whisk until smooth. The mixture will be very thin. Make sure there are no dry patches and the eggs are fully combined.
• Pour the filling into the baked crust.

Bake

• Return the pan to the center of the oven. Bake until just set (25 to 30 minutes).
• The filling is set when it does not jiggle more than a tiny bit in the center when the pan is shaken gently back and forth. It will also likely have begun to pulled away from the sides of the pan and should look uniform across the top.
• Remove the pan from the oven. Place the pan on a wire rack and allow the bars to cool, still in the pan, for about 20 minutes.

Chill the bars

• Cover the top of the pan with plastic wrap, and chill in the refrigerator until firm (at least 2 hours).
• Remove the bars from the pan by running a butter knife or thin spatula around the perimeter of the baking dish, then lifting the bars out of the pan by the overhung pieces of parchment paper.
• Pull the paper away from the sides of the baked bars to expose the sides. Use a very sharp knife to slice into 9 equal squares. Separate the bars from one another.
• Dust the top of the cut bars with confectioners sugar. Serve chilled.