In the bowl of a stand mixer fitted with the paddle attachment, combine the butter and sugars. Mix at medium speed for 2 to 3 minutes, or until light and fluffy. Add the egg and vanilla extract and mix until incorporated.

In a separate bowl, whisk together the flour, cinnamon, baking soda, and salt. Gradually add the dry ingredients to the butter mixture and mix until just combined.

Fold in the oats using a spatula or wooden spoon.

Using a medium spring-loaded cookie scoop, level, and roll about 1.5 tablespoons of the cookie dough and place it on one of the prepared baking sheets, spaced at least two inches apart. Cover and place in the refrigerator for at least 1 hour or overnight.

When ready to bake, preheat your oven to 350°F and line two baking sheets with parchment paper. Place the cookie dough on the prepared baking sheets about 2 inches apart.

Bake for 10-12 minutes, or until the edges are lightly golden. The centers should be slightly soft.

Remove from the oven and let the cookies cool on the baking sheet for 5 minutes before transferring them to a wire rack to cool completely.

While the cookies cool, make the icing. In a medium bowl, whisk the powdered sugar with the milk and vanilla until smooth and glossy.

Holding the cooled cookies upside down, dip the cookie tops in the icing. Allow cookies to sit for 10 to 15 minutes, or until the icing hardens and sets.