Begin by creaming together the softened butter, brown sugar, and granulated sugar in a mixing bowl until light and fluffy.

Add the vanilla extract and the egg to the creamed mixture, mixing well until fully incorporated.

In a separate bowl, combine the old-fashioned oats, all-purpose flour, cinnamon, nutmeg, baking soda, and salt. Stir until evenly mixed.

Gradually add the dry mixture into the wet mixture, stirring until just combined.

Preheat your oven to 350°F and prepare a baking sheet lined with parchment paper.

Drop heaping tablespoons of cookie dough onto the sheet, spacing them a couple of inches apart.

Bake until the edges are lightly golden and the centers appear set, approximately 10-12 minutes.

Remove from the oven and allow to cool on the baking sheet for a few minutes before transferring to a wire rack.

While the cookies cool, whisk together the powdered sugar, milk, and vanilla until smooth.

Once the cookies are completely cooled, drizzle or spread the icing over each cookie.