In a large bowl, sift together the flour and 3/4 teaspoon of the ground cinnamon and set aside.
Place the butter and confectioners' sugar in the bowl of a stand mixer fitted with the paddle attachment and cream until light and fluffy, about 4 minutes.
Add the yolks in a thin stream and mix until combined. Add the flour and cinnamon mixture and mix until just combined.
Shape the dough into a ball, wrap in plastic, and refrigerate for at least 2 hours and up to 2 days.
Preheat the oven to 300˚F on the convection setting. Line cookie sheets or sheet pans with parchment paper. Roll the dough to slightly less than 1/4 inch thick and use a 2-inch round cutter to cut cookies. Transfer to the prepared pans. Reroll scraps and cut again.
Beat the egg well and use it as an egg wash on top of the cookies, taking care to wipe the brush clean so that it doesn't drip down the sides.
Combine the granulated sugar and the remaining 3 tablespoons cinnamon and sprinkle the cinnamon sugar over the cookies.
Bake in the preheated oven until golden, 20 to 25 minutes. Cool the cookies on the pans on racks.