I first made these bars in the dead of winter when a bag of wild blueberries for the end of summer fell out of my freezer. I blind bake the cornmeal crust so that it doesn't get soggy from the berries as they bake.
In the summer, I simplify the recipe. I skip the bottom crust, double the berries and other filling ingredients (try raspberries), and top the whole thing with the brown butter crumb topping. And then I call it a crumble.
The cornmeal crust is adapted from Alice Medrich's lemon bars (in Chocolate and the Art of Low-fat Desserts). The idea of browning the butter for the topping comes from the blog "une gamine dans la cuisine."
If you've never browned butter - do it and then add it to anything. Brownies? Blondies? Pasta? Yes, yes, yes. —zahavah
Preheat oven to 350ºF and put a rack in the upper 1/3 of the oven. Grease an 8X8 square pan with vegetable oil or line it with parchment.
Mix together the crust. Stir together the flour, cornmeal, salt, and baking soda. Beat butter in another bowl until creamy (I use my mixer). Add the 1/3 C sugar and beat for about a minute until smooth. Beat in the egg, yogurt, and vanilla. Add in the dry ingredients and beat on low until just combined. The dough will be sticky, so this is going to get a little bit messy. Scoop the dough into the pan, spread it around evenly (I just did this with wet fingers), and prick all over with a fork. Bake for 15 minutes or until the top gets just starts to brown. Don't over-bake. Keep the oven on.
Make the filling. While the crust is baking, mix together the blueberries, sugar, cornstarch, and lemon zest (about 1 teaspoon) and juice (about 2 tablespoons).
Make the topping. To brown the butter, cook it in a saucepan over medium heat, stirring occasionally, until the butter starts to foam. After about 5 minutes, the butter will start to brown as the milk solids cook. Once you see little brown specks (the solids), take the butter off the heat. It should smell nutty but not burnt. In a bowl, mix together the brown sugar, flour, cornmeal, and salt. Then pour the browned butter over it and mix everything together. It should be a little clumpy.
When the crust comes out of the oven, pour the filling over the top (it should be a magenta color by now) and then crumble the topping over the blueberries. Bake for another 20-30 minutes until the topping sets — it won’t change color much, so you’ll have to test the firmness with your fingers.
It’s best to let bars to cool completely before cutting if you want everything to be neat. If you can’t wait, try to hold off for at least 5 minutes before digging in. Cut into squares (9 or 16, depending on how big you want them).