These bars really let the fresh blueberry flavor come through. The secret ingredient is just a touch of balsamic vinegar. My husband thought I had gone crazy, but you would never guess it is in there if no one told you. Actually, balsamic vinegar is a pretty common thing to add to strawberries and blueberries. It enhances their tartness, which is particularly good if you are starting with very sweet berries. If you are using very tart blueberries, you can omit the balsamic vinegar and still get a good result.
The bottom crust mixture (which is similar to a shortbread) is used as the topping as well, making these blueberry bars fairly quick and easy. One caveat - unlike other bars, these are not really finger food. Serve them on a plate with a fork since they tend to crumble a bit.
Preheat oven to 350 degrees.
1/3 Cup Confectioner's Sugar
1 Cup Butter
1 1/2 Cups Flour
Mix in food processor or by hand with a pastry blender. Set aside 1/2 Cup of this mixture to use as the topping on the bars. Press the remaining crust mixture into the bottom of an 8x8" square pan. Prick the crust all over with a fork, then bake for 12-15 minutes, until it is starting to get golden brown. Remove the crust from the oven and let it cool to room temp.
2 Cups Fresh Blueberries (washed and stems removed)
1/4 Cup granulated sugar (more or less to taste)
1/2 teaspoon balsamic vinegar
1/2 teaspoon ground cardamom
(just a pinch of cinnamon can be substituted here, but the taste will be a little different)
1 Tablespoon Cornstarch
Place about 1/2 cup of the berries with the sugar in a small saucepan. Heat on low, smashing up the berries with a fork or a potato masher. Continue cooking over low heat for a few minutes until the sugar is dissolved and the mixture is syrupy. Remove from heat. Add the balsamic vinegar and the cardamom, stir well. Let the mixture cool a bit, then add the remaining blueberries and the cornstarch. Stir well. Spread the blueberry filling over the cooled crust. Sprinkle the remaining crust mixture over the top. (Alternately, you could substitute a standard streusel topping or crushed almonds if you prefer.)
Bake at 350 degrees for about 30 minutes, until golden and bubbling. The filling should look "set" and no longer liquidy. You must let these bars cool completely or they will not cut well.