Definitely Not a Pop Tart

Stir water and cornstarch together. In a saucepan, heat the preserves or jam. Stir in the cornstarch mixture and simmer 2 minutes. Let cool.

Whisk together flour, sugar, and salt. Cut butter into the mixture to pea-sized crumbs. Whisk one egg and milk together and stir into the dough until it holds together. Divide dough in half, and form each into a roughly 4‑inch square. Wrap in plastic and chill for at least 30 minutes.

Let dough rest for 15 minutes at room temperature. Dust work surface generously with flour. Roll one dough slab out to just over 10×10½‑inches, about ⅛‑inch thick. Set aside, and roll out the second slab the same way. As a guide, lightly score pastry into six 3½×5‑inch rectangles. Beat one egg and brush over the dough sheet. Spread about 1½ tablespoons of filling in the center of each scored rectangle, leaving a ¼‑inch border.

Lay the second pastry sheet on top. Trim all the outside edges, and cut into six 3½×5‑inch rectangles. Separate the pastries, then press along each edge to seal. Decoratively press the tines of a fork along each edge. Transfer pastries to a parchment-lined baking sheet. Chill for 30 minutes.

Preheat oven to 350° F. Whisk a tablespoon of water into the remaining egg wash, and brush the tops of the chilled pastries. Use a fork to poke holes in the top layer of pastry to vent. Bake pastries for 30 to 35 minutes until lightly golden. Let cool completely on the baking sheet.

Stir frosting ingredients together until smooth. (Add milk as necessary to reach desired consistency.) Frost pastries, then optionally top with sprinkles.

Do not reheat in a toaster.