To make the pastry, sift the flour and salt into a large bowl. Add the chilled, cubed butter, and rub it in with your fingertips until the mixture forms pea-sized bits. You can also use a pastry blender for this step.

Mix the egg yolk with 1 tablespoon of cold water and add it to the flour-butter mixture. Use the pastry blender to mix until the dough just starts to come together. If needed, add more water a bit at a time.

Press the dough together, shape it into a disk, wrap it in plastic wrap, and refrigerate for at least 30 minutes. Keep the dough refrigerated until you're ready to prepare the rhubarb mixture.

To prepare the caramelized rhubarb filling, cut the fresh rhubarb into 1-2 inch pieces. In a deep 10-inch skillet with an ovenproof handle, melt the butter and sugar over medium heat. Arrange the rhubarb tightly on top, filling all gaps.

Cook over low heat for 20-30 minutes, until the rhubarb is lightly browned and the excess liquid has evaporated.

Preheat the oven to 375°F. Roll out the pastry dough on a lightly floured surface, making it slightly larger than the skillet. Cover the rhubarb mixture with the rolled-out pastry dough, pressing down around the edge to enclose completely.

Bake for 25-30 minutes until the pastry is golden brown. Remove from the oven and let it rest for 5 minutes. Cover the skillet with a serving plate and carefully flip it over.

Serve your Tarte Tatin with vanilla ice cream or whipped cream.