Add the butter, both sugars, and vanilla to a large mixing bowl. Using a hand mixer, beat them together until creamy.
1 stick butter, 1/2 cup sugar, 1/3 cup brown sugar, 1 tsp vanilla extract
Add the remaining cookie dough ingredients to the bowl and blend everything together until evenly combined into a stiff batter.
1/3 cup molasses, 2 tsp baking soda, 2 cups flour, 1 large egg, 1 tsp ground ginger, 1 tsp ground nutmeg
Spray an 8x8" baking dish with non stick cooking spray. Transfer the cookie dough to the prepared dish and spread out to all the edges in an even layer.
Bake at 350° for 20 minutes, then carefully remove the hot dish from the oven and let it sit and rest for 15-20 minutes.
Add all of the icing ingredients to a small bowl and whisk together until evenly combined. Spread the icing evenly out overtop of the dish, then finish it with some festive sprinkles on top.
1 cup powdered sugar, 1 - 1 1/2 tbsp milk, festive sprinkles
Slice into squares, serve, and enjoy!