For our gluten-free version of this classic bar cookie, we started with the topping. Plenty of lemon juice plus a whopping ¼ cup of lemon zest ensured plenty of bright flavor, while butter and heavy cream added richness. This filling also needed some form of thickener so that it could set up and slice neatly. Some recipes rely on flour, but we found that eggs delivered better results. Seven yolks plus two whole eggs gave our filling a smooth, custard-like texture that set up just enough without obscuring the pure, clean lemon flavor. Next, we moved on to the base. We knew we wanted something similar to shortbread: a crisp, buttery crust that could support the topping yet slice neatly and easily. We developed a simple shortbread crust using butter, sugar, our flour blend, and xanthan gum for our Gluten-Free Ginger-Fig Streusel Bars, and it worked perfectly here as well. To ensure the base didn’t become soggy, we prebaked the crust and then topped it with the custard and returned the pan to the oven. Letting the bars cool for the full 2 hours was key to ensuring the custard topping set up and was sliceable.
Ingredients
- 6 ounces (1⅓ cups) ATK All-Purpose Gluten-Free Flour Blend
- 2⅓ ounces (⅓ cup) granulated sugar
- ¼ teaspoon salt
- ¼ teaspoon xanthan gum
- 8 tablespoons unsalted butter, cut into ½-inch pieces and softened
- 2 large eggs, plus 7 large yolks
- 7 ounces (1 cup) plus 2 tablespoons granulated sugar
- ¼ cup grated lemon zest plus ⅔ cup juice (4 lemons)
- Pinch salt
- 4 tablespoons unsalted butter, cut into 4 pieces
- 3 tablespoons heavy cream
- Confectioners’ sugar
- Adjust oven rack to middle position and heat oven to 350 degrees. Make foil sling for 8-inch square baking pan by folding 2 long sheets of aluminum foil so each is 8 inches wide. Lay sheets of foil in pan perpendicular to each other, with extra foil hanging over edges of pan. Push foil into corners and up sides of pan, smoothing foil flush to pan. Spray with vegetable oil spray. For Filling
- Whisk eggs and yolks together in medium saucepan. Whisk in granulated sugar until combined, then whisk in lemon zest and juice and salt. Add butter and cook over medium heat, stirring constantly, until mixture thickens slightly and registers 170 degrees, about 5 minutes. Immediately strain mixture in fine-mesh strainer into bowl and stir in cream. Pour filling over cooled crust. Bake until filling is shiny and opaque and center jiggles slightly when shaken, 15 to 20 minutes, rotating pan halfway through baking. Let bars cool completely in pan on wire rack, about 2 hours. Using foil overhang, lift bars out of pan. Cut into 16 pieces and dust with confectioners’ sugar before serving. (Bars can be refrigerated for up to 2 days; crust will soften.)