Prep your pan and oven. Heat the oven to 350°F (175°C). Line a 9x13-inch pan with parchment, leaving overhang on the long sides for easy lifting. Lightly grease the parchment.

Make the crust. In a bowl, beat the softened butter with 1/3 cup monk fruit sweetener until creamy and slightly fluffy, about 1–2 minutes. Mix in vanilla if using. Add 2 cups flour and 1/4 teaspoon salt. Stir just until a soft dough forms.

Press and par-bake. Press the dough evenly into the prepared pan, getting it into the corners and smoothing the top. Prick a few times with a fork. Bake for 15–18 minutes, until edges are slightly golden and the surface looks set.

Whisk the lemon filling. While the crust bakes, whisk 1 cup monk fruit sweetener with the eggs until smooth and well combined. Add lemon juice, lemon zest, and milk. Whisk in 1/3 cup flour and 1/8 teaspoon salt until no dry spots remain. Don’t over-whisk—just mix until smooth.

Pour over hot crust. Give the filling a final stir. Pour it onto the hot crust immediately after it comes out of the oven. This helps the layers bond.

Bake until set. Return the pan to the oven and bake 18–22 minutes, or until the center is set and no longer jiggly. The surface may look slightly bubbly or glossy—that’s fine. Avoid overbaking; it can make the filling rubbery.

Cool completely. Let the bars cool in the pan on a rack until room temperature, then chill for at least 2 hours to firm up. This makes slicing cleaner.

Finish and slice. Lift the bars out using the parchment overhang. Dust lightly with powdered monk fruit sweetener. Use a sharp knife to cut into 20–24 squares, wiping the blade between cuts for neat edges.