Blueberries and corn are brilliant together. These blueberry bars incorporate the sweet crunch of cornmeal in the crust and topping along with the bright, tart notes that come from adding fresh lemon juice and zest to the blueberry filling.
Combine blueberries, sugar, cornstarch, lemon juice and salt in a 2-quart non-reactive saucepan over medium heat. Stir until mixture comes to a boil, reduce heat and continue to simmer stirring constantly until fruit thickens slightly. Remove from heat and add lemon zest.
Preheat oven to 350°F. Butter or grease a 9-inch square pan.
Stir together flour, cornmeal and salt and set aside.
Cream butter and sugar in the bowl of an electric stand mixer fitted with the paddle attachment until mixture is light and fluffy, and becomes pale in color. Add vanilla mixing quickly to incorporate.
Add dry ingredients on low speed being careful to scrape the sides of bowl several times. Stop mixing when ingredients are fully incorporated but dough is still crumbly. This happens quickly; the mixture will look dry and floury one minute, then little clumps will suddenly appear.
Evenly distribute slightly less than half of the crumbly dough (2 cups) over bottom of prepared pan and press lightly to hold in place. Put the remaining dough in the refrigerator.
Bake crust for 25 minutes, or until lightly toasty brown.
Spread 1-1/2 cups blueberry jam filling over crust and cover with remaining dough. An even covering will produce a crust that covers the jam almost entirely, letting it peak through here and there.
Bake until top crust is golden brown, about 40 minutes, rotating the pan halfway through the baking time. Remove from oven and cool on a rack for 10 minutes before running a knife around the edge of the pan to prevent the bars from sticking. Cool completely and cut into bars.