In a large mixing bowl, stir together the flour and sugar. Add the cold cubed butter and cut it into the flour mixture using a pastry cutter or fingertips until the texture becomes crumbly with small butter pieces throughout.

Add the egg yolk and two tablespoons of ice cold water. Mix until the dough starts coming together. Add another tablespoon of water if needed. Shape the dough into a flat disk, wrap it tightly, and chill in the refrigerator for 30 minutes.

Lightly flour the counter and roll the chilled dough into a circle about one-eighth inch thick. Carefully transfer the dough into a tart pan and gently press it into the bottom and sides.

Trim the extra dough from the edges and poke small holes into the bottom using a fork. Chill the crust again while preparing the filling.

Add the chopped rhubarb, sugar, and cornstarch into a large bowl. Stir well so every piece becomes coated evenly.

In another bowl, whisk together the egg, heavy cream, and vanilla extract until smooth.

Spread the rhubarb filling evenly into the prepared tart crust. Slowly pour the cream mixture over the rhubarb.

Gently tap the tart pan on the counter to help the filling settle evenly.

Preheat the oven to 375 degrees Fahrenheit. Place the tart onto a baking sheet and bake for 40 to 45 minutes.

The crust should become golden brown and the filling should look set around the edges with slight bubbling.

Remove the tart from the oven and cool on a wire rack for at least 30 minutes before slicing.

Serve warm, chilled, or at room temperature with whipped cream or vanilla ice cream.