Beat butter and sugars with an electric mixer until smooth. Add egg and vanilla, mixing well until combined, about 2 minutes. Stir in flour, baking powder, salt, and 1 1/2 teaspoons cinnamon with the mixer on medium speed. Beat in oats. Mix until dough comes together. Add in 1 tablespoon milk if the dough is crumbly. If dough is still crumbly, add milk 1 tablespoon at a time until it comes together. Fold in chocolate chips. Refrigerate for 30 minutes.