Position a rack in the center of the oven and preheat the oven to 350℉. Line two baking sheets with parchment paper.
In a large bowl, use an electric mixer to combine the butter, brown sugar and white sugar until completely incorporated, about 3 minutes. Beat in the egg, egg yolk, cinnamon, vanilla, baking soda and salt until completely smooth, about 1 minute.
Using a wooden spoon, stir in the flour and oats until no streaks of flour remain. Fold in the chocolate.
Using a medium cookie scoop, scoop about 1 ½ tablespoons of the dough and place it on one of the prepared baking sheets. Repeat with the remaining cookie dough, making sure the cookies are placed at least 2-inches apart.
Bake the cookies until they’re golden brown and firm around the edges, 9 to 11 minutes. Cook the cookies on the baking sheets before enjoying.