Work in the butter until the mixture holds together when you squeeze it, with pecan-sized lumps of butter still visible.
Mix the egg and milk, and add it to the dough, mixing just until everything is mixed.
Divide the dough in half; each half will weigh about 10 ounces.
Shape each half into a rough 3" x 5" rectangle, smoothing the edges.
Roll out immediately; or wrap in plastic, and refrigerate for up to 2 days.
Make the filling:
Whisk together the sugar, cinnamon, and flour.
Assemble the tarts:
If the dough has been chilled, remove it from the refrigerator and allow it to soften and become workable, about 15 to 30 minutes.
Place one piece on a lightly floured work surface, and roll it into a rectangle about 1/8" thick, large enough that you can trim it to an even 9" x 12".
(Laying a 9" x 13" pan atop the dough will give you an idea if you’ve rolled it large enough.).
Trim off the edges; place the scraps on a baking sheet, and set them aside, along with the 9" x 12" rectangle of dough.
Roll the second piece of dough just as you did the first.
Press the edge of a ruler into the dough you’ve just rolled, to gently score it in thirds lengthwise and widthwise; you’ll see nine 3" x 4" rectangles.
Beat the egg, and brush it over the entire surface of the dough.
Place a heaping tablespoon of filling into the center of each marked rectangle.
Place the second sheet of dough atop the first, using your fingertips to press firmly around each pocket of jam, sealing the dough well on all sides.
Press the tines of a fork all around the edge of the rectangle.
Cut the dough evenly in between the filling mounds to make nine tarts.
Press the cut edges with your fingers to seal, then press with a fork, to seal again.
Gently place the tarts on a lightly greased or parchment-lined baking sheet.
Prick the top of each tart multiple times with a fork; you want to make sure steam can escape, or the tarts will become billowy pillows rather than flat toaster pastries.
Refrigerate the tarts (they don’t need to be covered) for 30 minutes, while you preheat your oven to 350°F.
Sprinkle the dough trimmings with cinnamon-sugar; these have nothing to do with your toaster pastries, but it’s a shame to discard them, and they make a wonderful snack.
While the tarts are chilling, bake these trimmings for 13 to 15 minutes, till they’re golden brown.
Remove the tarts form the fridge, and bake them for 25 to 35 minutes, until they’re a light golden brown.
Remove them from the oven, and allow them to cool on the pan.
Variations: Instead of brown sugar and cinnamon, fill the tarts with a tablespoonful of chocolate chips.
Or with jam filling: 3/4 cup (8 ounces) raspberry jam.
1 tablespoon cornstarch mixed with 1 tablespoon cold water.
To make the filling, mix the jam with the cornstarch/water in a small saucepan. Bring the mixture to a boil, and simmer, stirring, for 2 minutes. Remove from the heat, and set aside to cool. Use to fill the pastry tarts.