1. Make the Flaky Pastry Dough:

• In a medium bowl, stir the all-purpose flour, 1 tablespoon sugar, and ½ teaspoon salt together with a fork or a whisk.
• Using the large holes of a box grater, grate the frozen stick of butter directly into the flour mixture.
• Using your fingertips or a pastry blender, quickly work the grated butter into the flour until the mixture resembles coarse crumbs with some small, pea-sized bits of butter still visible.
• Stir in the ice water, one tablespoon at a time, using a fork to gently incorporate it. The dough will still be rather shaggy and loose. Do not overwork it; visible bits of butter are key to flakiness.
• Turn the dough out onto a lightly floured surface. Use your hands to gently press and bring it together into a ball. When it holds its shape, form it into a flat disk.
• Wrap the dough disk tightly in plastic wrap and refrigerate for at least 1 hour, or overnight, to chill the butter and relax the gluten.

2. Cook the Rhubarb Filling:

• While the dough chills, make the filling. In a small saucepan, melt the 1 tablespoon of butter over medium heat.
• Add the chopped rhubarb and cook for about 3-5 minutes, stirring occasionally, until the rhubarb begins to soften.
• Stir in the 3 tablespoons of granulated sugar, the balsamic vinegar, lemon juice, and the pinch of kosher salt.
• Continue to cook, stirring, for another 3 minutes, or until the mixture becomes thick and jam-like.
• Remove the filling from the heat, transfer it to a small bowl, and refrigerate for at least 30 minutes to cool completely.

3. Roll and Assemble the Empanadas:

• On a lightly floured surface, roll out the chilled dough disk into a large circle or rectangle about ⅛ to ¼-inch thick.
• Using a 4-inch round cutter (or the rim of a wide glass), cut out as many dough rounds as you can. You should get about 6-7 initially.
• Gently gather the dough scraps, re-roll them once, and cut out at least one or two more rounds to get about 8 total.
• Place about 1 to 1.5 tablespoons of the cooled rhubarb filling in the center of each dough round.
• In a small bowl, whisk the egg and 1 teaspoon of water together to create an egg wash. Lightly brush the edges of each dough round with the egg wash.
• Fold one side of the dough over the filling to meet the other edge, creating a half-moon shape. Crimp the edges together firmly with your fingers or press them with the tines of a fork to seal.
• Place the assembled empanadas onto a parchment-lined baking sheet.

4. Chill, Prepare for Baking, and Bake:

• Refrigerate the baking sheet with the empanadas for 20 minutes to firm up the pastry again. Refrigerate the remaining egg wash.
• While the empanadas chill, preheat the oven to 400°F (200°C) with a rack placed in the center.
• In a small bowl, stir together the 1 tablespoon of sugar and ½ teaspoon of cinnamon for the topping.
• Remove the chilled empanadas from the refrigerator. Lightly brush the tops of each empanada with the egg wash.
• Sprinkle the cinnamon-sugar mixture evenly over the tops.
• Using the tip of a sharp knife, cut a small slit or hole on the top of each empanada to allow steam to escape.
• Bake for 20 minutes, or until the pastry is puffed and a beautiful golden brown.
• Remove the empanadas from the oven and let them cool slightly on the baking sheet before transferring to a wire rack. Serve warm.