The Best Churro Cookies (with dulce de leche filling)

0 Place your dulce de leche in the fridge to thicken for 24, preferably overnight or more (especially if your usual brand is very thin)
1 Line 2 large cookie sheets with parchment paper.
2 In a large mixing bowl with the paddle attachment, cream the unsalted butter, granulated sugar, brown sugar, and vanilla extract at medium-high speed for 1 ½ minutes using a stand mixer or hand mixer.
3 Mix in the egg and egg yolk until well combined.
4 In a separate bowl, whisk together the dry ingredients: flour, cornstarch, cinnamon, baking powder, and salt.Gradually add the dry ingredients to the wet mixture, mixing just until combined.
5 Scoop 1 tablespoon of cookie dough and roll it into small pieces, forming cookie dough balls. Make small wells in the top of each dough ball using the back of a teaspoon or your finger. Fill each indent with about ¾ teaspoon of chilled dulce de leche.Take another 1 tablespoon portion of dough, create an indent, and place it over the filled cookie. Gently press the edges to seal, then roll it into a small ball to smooth it out. Press it with the palm of your hand to shape it into a disc.
6 In a small bowl, combine granulated sugar and cinnamon for the cinnamon sugar coating. Roll each cookie dough ball in the cinnamon sugar mixture.
7 When ready to bake, preheat your oven to 350 F. Place the cookies on a prepared baking sheet, 3 inches apart. Bake for 10–13 minutes or until the cookies are golden brown on the edges and slightly soft in the center.Let the cookies cool completely on the pan.