Preheat oven to 350 degrees. In a large bowl, mix together the oats, brown sugar, flour, and baking powder. Add in the butter in pieces and mix until combined. In another bowl, toss the fruit with vanilla extract, sugar, and cornstarch.

Meanwhile, grease an 8x8 inch pan lined with parchment paper. Spread ⅔ of the oat mixture in the pan and punch it down into a thin crust. Add the fruit, and sprinkle with the remaining oat mixture, and bake for 40 minutes, or until crust is golden.

Let cool, cut into 9 bars, and serve either warm or cold.