Preheat the OvenPreheat your oven to 350°F (175°C) and grease or line an 8×8-inch baking pan with parchment paper.
Make the CrustIn a medium bowl, combine the flour and powdered sugar. Cut in the cold butter using a pastry cutter or fork until the mixture resembles coarse crumbs. Press evenly into the bottom of the prepared pan. Bake for 12–15 minutes or until lightly golden. Let cool slightly.
Prepare the Cream Cheese LayerIn a separate bowl, beat the cream cheese, granulated sugar, egg, and vanilla extract until smooth and creamy. Spread the mixture evenly over the warm crust.
Make the Rhubarb LayerIn another bowl, toss the chopped rhubarb with sugar, cornstarch, and lemon juice until well coated. Gently spread over the cream cheese layer.
Assemble the Crumb ToppingIn a small bowl, mix together the oats, brown sugar, flour, and cinnamon. Cut in the cold butter until crumbly. Sprinkle evenly over the rhubarb layer.
BakeBake the bars for 35–40 minutes, or until the topping is golden brown and the center is just set. Let the bars cool completely before slicing.
Chill & SliceFor clean cuts and the best texture, chill the bars in the fridge for at least 1 hour before serving.
