In KitchenAid or with an electric mixer, cream the butter. Add sugars and beat until light and fluffy.
Add egg, then egg yolks, one at a time, beating until completely combined and pausing to scrape down the sides of the bowl.
Stir in vanilla extract.
In medium-sized bowl, whisk together flour, baking soda, cornstarch, cinnamon and salt.
With mixer on low speed, gradually add flour mixture to butter mixture, pausing periodically to scrape down the sides of the bowl with a spatula to ensure all ingredients are well-combined.
Stir in oats, scraping down the sides of the bowl to ensure evenly combined.
Drop cookie dough by rounded spoonful (about 1 ½ Tbsp) onto ungreased or parchment paper lined cookie sheets, spacing about 2" apart.
Bake on 350F for 10-12 minutes. Allow cookies to cool on cookie sheet for 5 minutes before transferring to cooling rack to cool completely.
Be sure to allow cookies to cool completely before making into sandwiches.
Malted Chocolate Creme
With electric mixer, cream butter.
Gradually add powdered sugar until completely combined.
Add melted chocolate (make sure it is not too hot!) and beat until combined.
Add heavy cream and beat on high for 1-2 minutes, until well-combined.
Stir in vanilla extract and salt.
Add malted milk powder and beat on medium-high speed for one minute (pause to scrape down the sides to be sure it is well-combined).
Sort your cookies into like-sized pairs. Spread creme filling over the bottom of one cookie and sandwich it with the bottom of another cookie.