In a bowl, combine flour, sugar, and salt. Using a fork or pastry blender, work in the butter until it forms pea-sized crumbles. Whisk together egg and milk; stir into flour mixture with a fork until dough just comes together. Divide in half and shape into discs; wrap tightly in plastic wrap and refrigerate for at least 30 minutes or up to 2 days.
Preheat oven to 350 degrees F. Remove dough from the refrigerator and allow it to soften 10 to 15 minutes or until workable. Roll out on a lightly floured work surface to a rectangle about 1/8 inch thick and 9-by-12 inches in size. Using a ruler and a rotary or pizza cutter, cut into nine 3-by-4 inch rectangles. Arrange on a baking sheet lined with parchment paper or a silicone mat.
Beat the egg white with a splash of water, and brush it around the edges of each rectangle. Place a heaping tablespoon of filling into the center of each.
Roll and cut the second piece of dough just as you did the first. Top each filled pastry with a second rectangle of dough, using your fingertips to press firmly around all sides. Press and crimp edges with a fork or a ravioli cutter to seal.
Prick the top of each tart with a fork or make a few small slices with a pairing knife to allow the steam to escape when baking. Refrigerate for 30 minutes or freeze for 10 minutes, then bake for 25 to 35 minutes or until light golden brown. Remove them from the oven, and allow them to cool on the pan for 10 minutes before transferring to a wire rack to cool completely before glazing.
To prepare glaze, sift confectioners’ sugar into a bowl. Whisk strawberry jam with milk, then stir into sugar until it just flows off a spoon. Spread or drizzle onto cooled pastries. Sprinkle immediately with decorating sugar, if desired.
