Preheat your oven to 350°F (175°C). Grease a baking dish.
In a mixing bowl, combine flour, sugar, baking powder, and salt. Add cold butter and mix until crumbly. Press into the bottom of the prepared baking dish.
Bake the crust for 15 minutes.
In another bowl, mix rhubarb, sugar, cornstarch, lemon zest, and vanilla. Spread over the baked crust.
In a separate bowl, beat cream cheese, sugar, egg, and vanilla until smooth. Pour over rhubarb filling.
Bake for an additional 25 minutes or until set. Let it cool before slicing into squares.
Notes
Serve chilled or at room temperature. You can add a dollop of whipped cream on top if desired.