Preheat and prep: Heat the oven to 350°F (175°C). Line an 8-inch square pan with parchment, leaving overhang on two sides so you can lift the bars out later.

Make the crust: In a bowl, mix 2 cups almond flour, 1/3 cup powdered sweetener, and a pinch of salt. Stir in 6 tablespoons melted butter until sandy and evenly moistened.

Press and bake: Press the mixture firmly and evenly into the pan. Dock gently with a fork. Bake for 12–15 minutes, until lightly golden at the edges.

Whisk the filling: In a medium bowl, whisk 3 large eggs until smooth. Add 1/2 cup powdered sweetener, 1/2 cup fresh lemon juice (about 3–4 lemons), 1–2 teaspoons finely grated lemon zest, 1 teaspoon vanilla (optional), 1 tablespoon cornstarch or arrowroot, and a tiny pinch of salt. Whisk until no lumps remain. The mixture should be silky.

Strain for smoothness (optional but recommended): Pour the filling through a fine-mesh sieve into a pouring jug to remove any zest clumps or eggy bits. This gives a clean, custardy texture.

Lower the oven temp: Reduce oven heat to 325°F (165°C). This helps the filling set gently and prevents curdling.

Fill and bake: Pour the lemon mixture over the hot crust. Bake 16–22 minutes, until the center is just set and barely jiggles. Do not overbake.

Cool completely: Let the pan cool to room temperature, then chill for at least 2 hours. Cold bars slice neatly and taste brighter.

Finish and slice: Lift out using the parchment. Dust lightly with powdered sweetener if you like. Use a sharp, warm knife (wipe between cuts) to slice into 16 squares.

Serve: Enjoy chilled or at cool room temperature. The flavor peaks on day two.