Step 1: In a medium bowl, whisk together the all-purpose flour, ground ginger, ground cinnamon, baking soda, salt, ground cloves, and ground nutmeg. Set aside.
Step 2: In a large bowl, using an electric mixer, cream together the softened unsalted butter, brown sugar, and granulated sugar until light and fluffy, about 2-3 minutes.
Step 3: Beat in the molasses, large egg, and vanilla extract until well combined and smooth.
Step 4: Gradually add the dry ingredient mixture to the wet ingredients, mixing on low speed until just combined. Be careful not to overmix.
Step 5: Fold in the old-fashioned oats until evenly distributed throughout the dough.
Step 6: (Optional but recommended) Cover the dough with plastic wrap and chill in the refrigerator for at least 30 minutes to 1 hour. This helps prevent spreading.
Step 7: Preheat your oven to 350°F (175°C). Line baking sheets with parchment paper.
Step 8: Scoop rounded tablespoons of dough (or use a 1.5-inch cookie scoop) and place them about 2 inches apart on the prepared baking sheets. Flatten them slightly with the palm of your hand.
Step 9: Bake for 10-12 minutes, or until the edges are set and lightly golden, but the centers still look soft. Do not overbake. Allow cookies to cool on the baking sheet for 5 minutes before transferring to a wire rack to cool completely.
Step 10: While the cookies are cooling, prepare the icing. In a small bowl, whisk together the powdered sugar, 2 tablespoons of milk or water, and 1/2 teaspoon of vanilla extract (if using) until smooth. Add more liquid a teaspoon at a time if needed, until you reach a pourable but thick consistency.
Step 11: Once the cookies are completely cool, drizzle or spread the icing over each cookie. Let the icing set completely before storing (this may take 30 minutes to an hour).