1. Grease and line a 8″x8″ baking pan with parchment paper, making sure that the parchment paper hangs over the edge of the pan so that it’s easy to lift and remove the bars from the pan. Preheat the oven to 350° F.

The Crust & Topping

1. Whisk together the flour, oats, brown sugar, baking powder, salt, and cinnamon in a bowl until combined. Add in the melted butter and vanilla extract and mix until the dough comes together.
2. Take about 2/3 of the dough and drop it into the lined baking pan. Press down on the dough with your fingers so that it’s in a flat, even layer covering the entire pan. Set aside the remaining 1/3 of the dough.
3. Bake the crust for 10 minutes. Then, remove to slightly cool.

The Triple Berry Filling

1. Measure out the cornstarch into a small bowl and set aside.
2. In a small to medium sized pot, add the frozen berries, lemon juice, and sugar and cook on low heat until the berries start to release some of their juice.
3. Take about 1/4 cup of the juice and add it to the bowl with the cornstarch. Stir until the cornstarch is dissolved, and then add the mixture back into the pot with the berries.
4. While stirring occasionally, let the mixture come to a low boil and cook for about 10 minutes. The filling will become thicker.
5. Evenly spread the filling on top of the warm crust. Crumble the dough that was set aside so that it’s in small pieces. Sprinkle and gently press it on top of the filling.
6. Bake the bars for 40-50 minutes. The topping should be golden brown and the filling will be bubbling. When done, place the pan on a wire rack to cool. Once completely cooled, remove the bars from the pan, cut, and serve.