1 ½cupheat-treated all-purpose flourIf you can't find heat treated flour in the store, follow my instructions in my edible cookie dough post to heat treat your own.
Instructions
Cookies
Preheat oven to 400F and line cookie sheet with parchment paper.
In stand mixer (or in large bowl using hand mixer) beat together sugar and shortening on high speed until light and fluffy.
Beat in egg and egg yolk, pausing to scrape down sides and bottom of bowl, until well-combined, and then stir in vanilla extract.
In separate bowl, combine flour, cocoa powder, baking powder, baking soda and salt.
Measure out milk and set aside.
Starting with flour-mixture, gradually stir flour mixture and milk into batter, alternating (add about ⅕ of dry mix, as soon as ingredients are combined stir in approximately ⅕ of the milk, and repeat until all ingredients are combined).
Drop batter by 1 ½ Tbsp-sized spoonfuls onto prepared cookie sheet, spacing at least 2" apart.
Bake 8 minutes, remove from oven and allow to cool at least 5 minutes before transferring to cooling rack to cool completely. Allow shells to cool completely before filing.
Cookie Dough Filling
In KitchenAid (or with electric hand mixer) cream together butter and shortening on medium-speed until well combined (at least one minute).
Add sugars and salt and beat until light and fluffy.
Stir in vanilla extract, pause to scrape down sides and bottom of bowl with rubber spatula.
Add 1 cup of the flour and stir until combined.
Add milk, stir until combined, and then add the remaining ½ cup of flour, stirring until combined (scrape down the sides of the bowl to make sure well combined).
Spread frosting onto bottom of one cookie and then sandwich with another evenly matched (similar sized) cookie. Alternatively you may fill a piping bag fitted with a large tip with the frosting and pipe it onto the cookies, which is what I did here).
Refrigerate uneaten cookies in a sealed container.