• Preheat oven to 350°F with racks near the center. Line two baking sheets with parchment paper and set aside.

Cinnamon sugar filling:

• Mix together all ingredients (I use a small rubber spatula or fork) until well combined. Place in the freezer for about 10 minutes while you prepare the cookie dough.

Cookies:

• Using an hand mixer, or stand mixer fitted with the paddle attachment,  beat butter and sugar until light and creamy. Add vanilla, egg, and egg yolk, and beat until combined, scraping the sides of the bowl as necessary.
• In a separate bowl, combine flour (see note), baking powder, cornstarch, and salt. Slowly add the dry ingredients to the wet ingredients and beat until combined.
• Remove half of the dough from the mixing bowl and spread the remaining dough out across the bottom the mixing bowl.

  Remove the cinnamon mixture from the freezer and use two spoons to scoop small pieces (about a teaspoon each) of the cinnamon mixture and place them all over the dough in the mixing bowl.

  Once you have used half of the cinnamon mixture, get the cookie dough that you have set aside, and spread it or press it with your fingers into a somewhat even layer on top of the cinnamon and dough in the mixing bowl.

  Proceed to add the remaining cinnamon mixture (in little pieces) all over the top of that piece of dough. Your bowl should now have two layers of cookie dough, each layer topped with cinnamon sugar bits.
  
• Once you have your dough layered you are going to fold and mix it just barely. I do two folds, scraping up and over, followed by one to two stirs by hand with a rubber spatula. Do not use an electric mixer, this needs to be a brief stir that leaves some pieces of the cinnamon mixture in chunks and some pieces swirled into the dough.
  
• Scoop the dough into two-tablespoon portions using a cookie scoop. Pay attention while scooping to ensure that each scoop has a fair amount of dough mixed with cinnamon.

  Place the dough balls on a plate and freeze for 10 minutes. This quick chill firms up the butter so the cookies don't spread too much.
  
• Place dough balls a couple inches apart on parchment lined baking sheets. For thicker cookies leave the dough balls in higher mounds, for thinner cookies give them a slight pat down before baking. Any dough balls that do not fit into this batch that's baking should be refrigerated (not frozen) until the baking sheets have cooled and you're ready to bake them.
• Bake for 10-12 minutes, until the cookies have spread, the edges are beginning to brown and the center is still slightly underdone.

  Some of the cookies will bake up a little misshapen. This is because pockets of cinnamon sugar might melt out while baking. Don't worry, I have two techniques that work for getting a perfectly round shape for every cookie in the batch! See notes for shaping cookies as soon as they come out of the oven.

  Place baking sheets on wire racks and allow cookies to cool completely. They will firm up as they cool.