Preheat the oven to 350°F. Line two baking sheets with parchment paper.
In a large bowl, beat the softened butter, brown sugar, and granulated sugar for 2–3 minutes, until light and fluffy.
Mix in the egg, then the egg yolk, then the vanilla. Scrape down the sides of the bowl.
In a separate bowl, whisk together the flour, baking soda, cinnamon, and salt.
Add the dry ingredients to the wet ingredients and mix just until combined. Do not overmix.
Cover and chill the dough for 30 minutes (recommended for thicker cookies).
In a small bowl, mix the cinnamon-sugar coating ingredients.
Scoop the dough into 2-tablespoon portions and roll into balls. Roll each ball in the cinnamon sugar.
Place dough balls on the prepared baking sheets, leaving space between each.
Bake 9–11 minutes, until edges are set and centers still look slightly soft.
Cool on the baking sheet for 5 minutes, then transfer to a cooling rack.