In large bowl (a stand mixer works best) beat butter, brown sugar, and powdered sugar until well mixed.
Add egg yolk, vanilla, and milk, and beat well.
Add graham flour, all-purpose flour, and cornstarch and beat on low until combined. (This is a very stiff dough, so you may need to use a dough hook to finish mixing.)
Roll out on lightly floured surface. These should be fairly thin - no more than ¼-inch. Cut out desired shapes with cookie cutters.
Place close together on baking sheet. Bake 9-10 minutes. Let them cool on the sheet for a few minutes and then transfer to a rack to cool completely.
Recipe by The Rowdy Baker at http://www.therowdybaker.com/?p=955