Preheat your oven to 350°F. In a medium bowl, whisk together the flour and 1/4 cup sugar. Cut the cold butter into small cubes and work it into the flour mixture using a pastry cutter, two knives, or your fingertips until the mixture resembles coarse breadcrumbs with some pea-sized pieces of butter remaining. Press this mixture firmly and evenly into a 9x13-inch baking dish, making sure to create an even layer across the bottom. Bake for 10 minutes until the crust is set but not yet golden—this par-baking prevents a soggy bottom once the custard is added.

While the crust bakes, prepare the custard layer. In a large bowl, whisk together 2 cups sugar, 7 tablespoons flour, and 1/4 teaspoon salt. In a separate small bowl, whisk the 3 eggs until blended, then add 1 cup heavy cream and stir until combined. Pour the wet mixture into the dry ingredients and whisk until smooth and well incorporated. Gently fold in the 5 cups of prepared rhubarb pieces until evenly distributed throughout the custard. I like to fold gently here to avoid breaking down the rhubarb too much, which keeps the bars from becoming mushy.

Remove the par-baked crust from the oven and immediately pour the custard-rhubarb mixture over it, spreading it evenly. Return to the oven and bake for 40-45 minutes until the custard is just set but still slightly jiggly in the very center—it will continue to set as it cools. The edges should be golden and the filling should no longer look wet on top, but overbaking will make the custard tough and grainy.

While the bars finish baking, make the topping. Remove the 8 oz cream cheese from the refrigerator if it hasn't already reached room temperature (about 70°F)—this ensures it will blend smoothly without lumps. In a medium bowl, beat the softened cream cheese with 3/4 cup sugar and 1/2 teaspoon vanilla until light and fluffy, about 2-3 minutes. In a separate bowl, whip the remaining 1 cup heavy cream to stiff peaks. Gently fold the whipped cream into the cream cheese mixture in two additions, being careful not to deflate the whipped cream too much.

Remove the custard bars from the oven and let them cool to room temperature, about 30 minutes. This cooling step is important—if you spread the topping on hot bars, it will melt and become too soft. Once cooled, spread or dollop the cream cheese whipped cream topping evenly over the bars. Refrigerate for at least 2 hours before serving to allow the topping to set and the flavors to meld. I find these bars taste best served chilled, and they hold their shape much better this way.