Rhubarb Bars

Preheat the oven to 375°F. Butter 9 x 9 baking dish. Line the pan with a piece of parchment paper, cut so that it extends on two sides to easily remove it from the pan.

Place the rhubarb, granulated sugar, cornstarch, lemon zest, and vanilla extract in a saucepan. Cook on medium heat for 5 minutes, stirring often, until it thickens into a sauce. Set aside.

In a medium bowl, mix the flour, brown sugar, oats, cinnamon, and salt. Cut in the butter with a pastry cutter or fork. As it incorporates, use fingers to knead until it becomes an even, consistent dough with no more dry bits of flour (this takes a while, so be patient!). Use your fingers to crumble it into small crumbles and measure out 1 ¾ cup for the topping.

Pour the remaining crumbles into the prepared pan, and press it into an even layer for the crust of the bars. Use the bottom of a measuring cup to press in the dough and make it flat and even.

Spoon the rhubarb filling onto the top of the crust. Add the reserved crumbles over the top in an even layer.

Bake for 35 to 40 minutes, until golden brown. Remove from the oven and cool on cooling rack for 30 minutes. Then transfer to the refrigerator and for 1 hour. Remove from the fridge and use the parchment to remove the bars from the pan. Cut into 16 even squares. Store bars up to 1 week refrigerated (with parchment paper between the layers if stacked), or 3 months frozen.