Step 1: Preheat your oven to 350°F (175°C). Lightly grease a 9x13-inch baking pan or line it with parchment paper.
Step 2: In a medium bowl, combine 1½ cups all-purpose flour and ½ cup powdered sugar. Cut in ¾ cup cold, cubed unsalted butter using a pastry cutter or your fingers until the mixture resembles coarse crumbs.
Step 3: Press the crumb mixture evenly into the bottom of the prepared pan, forming a compact layer. Bake for 15 minutes or until the edges are lightly golden. Remove and set aside to cool slightly.
Step 4: While the crust cools, prepare the cream cheese layer. In a mixing bowl, beat 8 oz softened cream cheese with ½ cup granulated sugar until smooth and creamy.
Step 5: Add 1 large egg and 1 tsp vanilla extract to the cream cheese mixture. Mix until fully combined and lump-free.
Step 6: Pour and gently spread the cream cheese mixture evenly over the warm, pre-baked crust.
Step 7: In a small saucepan, combine 2 cups diced fresh rhubarb, ½ cup granulated sugar, 1 tbsp cornstarch, and ¼ cup water.
Step 8: Cook over medium heat, stirring frequently, until the mixture thickens and the rhubarb is softened (about 5-7 minutes). Let it cool slightly.
Step 9: Gently spoon the cooled rhubarb mixture over the cream cheese layer, spreading it evenly.
Step 10: Prepare the crumble topping: In a small bowl, mix ½ cup all-purpose flour and ½ cup packed brown sugar. Cut in ¼ cup cold, cubed unsalted butter until the mixture is crumbly.
Step 11: Sprinkle the crumble topping evenly over the rhubarb layer.
Step 12: Return the assembled bars to the oven and bake for 30-35 minutes, or until the top is golden and the center is set with a slight jiggle.
Step 13: Allow the pan to cool completely at room temperature, then chill in the refrigerator for at least 2 hours before cutting into bars. For best results, chill overnight.