Preheat oven to 350° F. Line a baking sheet with parchment paper and set aside. In a small bowl, mix together the sugar and cinnamon and set aside.
In a medium bowl, whisk together the flour, baking powder, and salt. Set aside.
In a large bowl, cream the butter and sugar until light and fluffy, about 3-4 minutes. Add in the egg, egg yolk, and vanilla, and mix until smooth. Scrape down the sides of the bowl with a rubber spatula when necessary.
Slowly add the dry ingredients to the wet and mix until no streaks of flour are left.
Use a medium cookie scoop (about 1½ tbsp) to scoop the cookie dough and roll it into a ball using your hands. Roll the ball in the cinnamon sugar mixture. Roll the dough in your hands again (to ensure the coating sticks to the dough), and roll in the cinnamon sugar one more time. Place on the cookie sheet.
Bake cookies for 9-11 minutes, or until just set. Remove from the oven and let them cool on the cookie sheet for about 5 minutes, and then remove onto a wire cooling rack to cool completely.