In a large bowl, whisk together the flour, sugar, salt and cinnamon until well-combined. Add in the cold butter and work in with your hands or a pastry blender until coarse crumbs, the size of peas, form. Try to work fast so as to not warm up the butter too much.

In a small bowl, whisk together the egg yolk and milk. Pour into the dry ingredients and stir with a rubber spatula until the mixture comes together to form a shaggy dough. Bring together onto a sheet of plastic and wrap, forming it into a disc. Chill for at least 1 hour or overnight.

Preheat oven to 400°F. Line two baking sheets with parchment paper and set aside.

Unwrap the dough and place on a lightly floured work surface. Roll out to a large rectangle about 1/2-inch thick. Using a pizza cutter or sharp knife, trim the edges, and then cut into 2x1-inch rectangles. Place on the prepared baking sheets, close together. Using a fork, poke holes to allow steam to escape. Brush with egg wash and bake until golden brown and flaky, about 15 to 18 minutes. Remove from the oven and allow to cool completely.

In a medium bowl, whisk together the powdered sugar, vanilla and enough milk to create a smooth pourable glaze. If it's too thick, add a splash more milk until desired consistency is reached. Color with food coloring if desired. Dip the top of each cooled cookie into the glaze, carefully so as to not crush them. Top with sprinkles while still wet. Allow the glaze to set before eating. Can be frozen on baking sheets and then transferred to freezer bags for later! Enjoy!