01 - Preheat the oven to 190°C (375°F).
02 - In a bowl, mix the flour, softened butter, powdered sugar, and egg yolk until the mixture becomes crumbly. Gradually add cold water, one tablespoon at a time, until the dough comes together.
03 - Press the dough evenly into a 9-inch tart pan and prick the bottom with a fork to prevent bubbling. Bake for 15 minutes, then remove from the oven and let it cool.
04 - In a saucepan over medium heat, combine rhubarb and ½ cup of sugar. Cook until the rhubarb softens, about 5 minutes. Stir in the cornstarch and remove from heat.
05 - In a separate bowl, whisk together the remaining sugar, eggs, vanilla extract, and heavy cream until fully combined.
06 - Pour the rhubarb mixture into the cooled tart crust, then gently pour the custard mixture over the top.
07 - Bake the assembled tart for 25-30 minutes, or until the custard is set and lightly golden on top.
08 - Allow the tart to cool completely before slicing and serving.