To 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. Wrap in plastic or waxed paper, and refrigerate for 30 minutes; this will relax the gluten and firm up the dough a bit, making it easier to roll. You can also refrigerate the dough for up to 2 days; let it rest for 15 to 20 minutes at room temperature before you roll it out.
To make the filling: Whisk together the brown sugar, flour, and cinnamon.
To assemble the tarts: Place one piece of the dough on a generously 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.