In a medium bowl, whisk together flour, cinnamon, ginger, allspice, cloves, baking soda, and salt to combine. Set aside.

In a large bowl, cream together butter, brown sugar, and granulated sugar until you no longer see any dry sugar crystals.

Whisk the egg into your butter/sugar mixture until combined. Whisk in molasses and vanilla to combine.

Add the dry ingredients and fold together until just combined. Take care not to over mix or the cookies will end up hard.

Spread the cookie dough into your prepared baking pan. Bake the cookie in the center of your preheated 350°F oven for 20 to 25 minutes, or until no longer shiny on top and the edges are slightly browned. Take the baked cookie out of the oven and set aside to cool completely in the pan.

In a medium bowl, whisk together powdered sugar and 1 tablespoon of milk. If needed, gradually whisk in more milk until you reach your desired icing consistency.

Pull the cooled cookie out of the pan using the parchment overhang and place on your serving platter. Pour the icing overtop. Spread icing evenly to cover the top of the cookie.

Let sit for 5 to 10 minutes, or until a light crust forms on the surface of the icing before decorating with sprinkles and/or gingerbread cutout cookies! Decorating the cookies before the icing has a chance to set slightly will cause the toppings to slip, slide and sink! But be careful not to wait too, too long or the icing will set and your decorations won't stick.

After decorating, let the cookie sit until the icing is fully set for cleaner slices. You could slice it right away, but it'll get messy!