Make the dough: Whisk together the flour, sugar, and salt. 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 cohesive.
Divide the dough in half. 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".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 to gently score it in thirds lengthwise and widthwise. You should end up with 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 to let steam escape. Refrigerate for 30 minutes, while you preheat your oven to 350F.
Remove the tarts form the fridge, and bake them for 25 to 35 minutes, until they are a light golden brown. Remove them from the oven, and allow them to cool on the pan.