To make the dough:
Whisk collectively the flour, sugar, and salt. Work within the butter till the combination holds collectively once you squeeze it, with pecan-sized lumps of butter nonetheless seen. Combine the egg and milk, and add it to the dough, mixing simply till every thing is cohesive.
Divide the dough in half. Form every half right into a tough 3″ x 5″ rectangle, smoothing the perimeters. Wrap in plastic or waxed paper, and refrigerate for half-hour; it will loosen up the gluten and agency up the dough a bit, making it simpler to roll. You can even refrigerate the dough for as much as 2 days; let it relaxation for 15 to twenty minutes at room temperature earlier than you roll it out.
To make the filling: Whisk collectively the brown sugar, flour, and cinnamon.
To assemble the tarts: Place one piece of the dough on a generously floured work floor, and roll it right into a rectangle about 1/8″ thick, giant sufficient you could trim it to a fair 9″ x 12″. Trim off the perimeters; place the scraps on a baking sheet, and set them apart, together with the 9″ x 12″ rectangle of dough.