Preheat the oven to 350°F (175°C) and line two large baking sheets with parchment paper.

In a stand mixer or large bowl, cream together the softened butter, brown sugar, and granulated sugar until the mixture is light and fluffy, approximately 3 minutes.

Add the eggs one at a time, beating well after each addition, then stir in the 1 tablespoon of vanilla extract.

In a separate medium bowl, whisk together the flour, ground cinnamon, baking soda, and salt.

Gradually add the dry flour mixture to the wet butter mixture, mixing on low speed until just combined.

Fold in the rolled oats by hand or with a spatula until evenly distributed throughout the dough.

Use a cookie scoop to form 1.5-inch balls of dough and place them on the prepared baking sheets, spaced 2 inches apart.

Bake for 10 to 12 minutes, or until the edges are golden brown but the centers still appear slightly soft.

Cool the cookies on the baking sheet for 5 minutes before transferring them to a wire rack to cool completely.

Prepare the glaze by whisking the powdered sugar, milk, and 1/2 teaspoon vanilla extract in a small bowl until smooth.

Drizzle the glaze over the cooled cookies in a spiral or zig-zag pattern and allow it to set before serving.