Prepare the filling. In a medium saucepan, combine the berries, juice, zest, vanilla, sugar and salt. Cook over medium-low heat, covered until the berries are soft and begin to burst. Crush as many berries as you can with the back of a wooden spoon or a rubber spatula.

Continue cooking, stirring often, until the berries have broken down completely and you’re left with a sauce-y, thickened mixture. Stir in the corn starch-water mixture and let the berry filling come to a boil.

Cook until the mixture has thickened more and a spoon leaves a trail on the bottom of the pan that lasts for a second or two. Remove from heat and set aside to cool.

Prepare a 9x9-inch tin by greasing it with butter and lining it with parchment paper. Set aside.

Make the crust and topping. In a food processor, stand mixer fitted with the paddle attachment, or in a large bowl with a hand mixer, beat the butter, sugar and lemon zest until smooth and creamy.

Beat in the egg and vanilla. The mixture may curdle slightly — that’s okay. Beat in the salt and *only 2 1/4 cup* of the flour just until the last streaks of flour disappear. Spread half of the dough into the bottom of the tin in one even layer.

Add the remaining 1/4-cup of flour to the other half of the dough and fold in just until combined. Roll out the dough between two sheets of parchment paper into a rectangle that measures about 9x13 inches. Chill the dough in the fridge or freezer until it is completely firm.

If at any point the dough becomes too warm and slack, just stick it back in the fridge or freezer on a flat surface. It’s much better to be patient than try to fight loose dough. Cut the dough into about 24 strips, or as many as you can, each a half-inch thick and at least 9 inches long. (You want to have extra in case of breakage.)

On a sheet of parchment or a silicone mat, arrange 10 strips, evenly spaced apart, in a vertical lines so that they are 9 inches from end to end.

Starting from left to right, fold the first strip up halfway, then do the same for the third, fifth, seventh and ninth. Lay dow a strip of dough horizontally, right where the fold is, then bring the strips you had folded up down again over top of the horizontal strip.

Now fold up vertical strips 2, 4, 6, 8 and 10. Repeat the same step as above with another new strip going horizontally, and the fold the vertical strips down again.

Repeat these steps with three more horizontal strips, alternating folding up every other vertical strip, to complete the bottom half.

Turn the unfinished top half of the lattice to face you and repeat the weaving pattern on that side with five more horizontal strips. Transfer the lattice to the freezer until completely firm.

Preheat oven to 400°F. Spread the blueberry filling evenly onto the dough in the tin. Then delicately place the lattice top on top of the filling. You may have to trim it to fit snugly in the tin. Transfer to the freezer once more to completely firm up before baking.

Bake on the middle rack at 400°F for 10 minutes, then turn the oven down to 350°F and bake for another 30 minutes. The filling should be bubbling on the sides and under the crust, and the lattice should be starting to lightly brown on the high points.

Allow the bars to cool completely in their tin. For clean slices, chill the bars in the fridge for a couple hours, or for pristine slices, stick them in the freezer. Use a long, sharp, serrated knife to delicately score through the tops then press through to the bottom.