Chocolate Shortbread Cookies

In a medium mixing bowl fitted with the paddle attachment, blend together butter and sugar for 2-3 minutes on medium until pale and fluffy. Add in the vanilla and egg yolk and mix again.
Whisk together the salt, flour and cocoa powder. Add into the wet ingredients and mix until completely incorporated. The dough may seem a bit crumbly, but it will come together.
Roll out to be about ½ inch thick or however you like your cookies. Use a cookie cutter and cut dough, placing on a parchment lined baking sheet as you go. The cookies do not spread, so you can place them close together. Once all dough has been cut, place cookie sheet in the fridge for 2 hours to allow dough to chill. They can refrigerate longer if you want, just make sure to cover them well so they don't dry out.
Preheat oven to 325 F. Roll chilled cookie dough in extra granulated sugar and bake for 14-17 minutes. You want just the edges to start to turn a light brown. Allow to cool completely before dipping in chocolate.
Melt chocolate in the microwave starting with 30 seconds, stir, and then heat again, stirring in 30 second intervals until completely melted and pourable. Pour into a bowl and dip the side of each cookie into the chocolate. I keep a little spoon or spatula near it to scrape off excess chocolate that could drip or puddle. Sprinkle with salt flakes and let the cookies set up.