Line an 8x8 inch baking pan with foil.
Put the flour, sugar, cornmeal, cornstarch and salt in a food processor and pulse a few times to blend. Add our cubes of butter a few at a time until you have a moist dough that forms big crumbs. Dump the dough into the baking dish and press into the bottom of the pan - as evenly as you can. .
Center a rack in the oven and preheat it to 350 degrees F. While the oven is preheating, put the dish in the freezer or fridge to keep the butter in it cold.
Bake the crust for 25 minutes, until it’s golden brown - and I mean golden, like, more than you would think for pre-baking. You want the crust crisp and able to hold up to a custard and the juice expelled from the blueberries, so make sure you cook it long enough. Put the pan on a cooling rack and cool it completely.
Keep the oven on or make sure to reheat it again to bake the bars.
Place the cornstarch in a small bowl and pour over 1/4 cup of the buttermilk. Stir it until the cornstarch dissolves.
Whisk the eggs in a medium bowl until foamy. Add the sugar and immediately start whisking vigorously (you must beat sugar and eggs together quickly, or the sugar will “burn” the yolks and cause a film to form). Whisk in the salt and vanilla, then whisk in the slurry. When the slurry is fully incorporated, stir in the remainder of the buttermilk, followed by the melted butter. Scatter the blueberries over the crust and then pour on the topping. The blueberries will move all over the place when you pour the custard in, so try to push them back around to distribute evenly if you can, this can get annoying so don't sweat it.
Bake the bars for 42 to 45 minutes, until the topping is puffed all the way to the center, brown around the edges and firm everywhere. Transfer the pan to a rack and cool for 20 minutes at least. Pull the foil and lift out of the pan to finish cooling. Then cut into bars, plate them up and enjoy.