• Combine flour, sugar and salt in a food processor; pulse a few times. Sprinkle butter over top and pulse until butter pieces are about the size of coarse crumbs. Scatter ice water over and pulse again a few times until a shaggy dough forms.
• Turn dough out onto a floured counter. Gather the dough into a cohesive lump and pat it into a disc. Wrap dough in plastic wrap and refrigerate overnight, or for at least two hours.

To assemble:

• Preheat oven to 400º F. Line two baking sheets with parchment paper.
• Roll dough into a rectangle (approximately 16 by 11" wide). Trim edges to make straight lines. Use a knife or pastry wheel to cut dough crosswise into 6 equal strips.
• Divide the rhubarb filling among the dough strips, placing it in a small pile on one end of each rectangle, then fold remaining dough over and pinch edges to seal. Use a sharp paring knife to cut two 1" slits in the tops of each pie. Transfer pies to parchment-lined baking sheet.
• In a small bowl, whisk egg with 2 tsp water. Brush pies with the egg wash. Bake pies for about 25 minutes, until golden brown and filling is bubbling out.

To make icing:

• Whisk together confectioners’ sugar, vanilla extract and water. Drizzle over warm pies in a zigzag fashion.