In a large bowl, combine the flour and sugar. Cut in the cold cubed butter until the mixture resembles coarse crumbs.

Stir in the egg yolk and add ice water a tablespoon at a time until the dough holds together. Wrap it in plastic wrap and refrigerate for at least one hour.

For the frangipane, blend the finely ground pistachios with sugar, softened butter, egg yolk, and vanilla extract until smooth and creamy.

Preheat your oven to 375°F (190°C). On a lightly floured surface, roll out your chilled dough into a 12-inch circle. Transfer it to a parchment-lined baking sheet.

Spread the pistachio frangipane evenly over the center of the dough, leaving a two-inch border around the edges. Arrange the sliced rhubarb on top.

Gently fold the edges of the dough toward the center, partly covering the filling. Brush the exposed edges with an egg wash. Bake for 35-40 minutes until the crust is golden brown and the rhubarb is bubbling.