Place the butter in a 10-12 oz. mug. Microwave on low until melted. Stir in the crushed gingersnap cookies and press firmly to the bottom of the mug to make a crust. Set aside.

In a small bowl, whisk together the pumpkin purée, egg, milk, brown sugar, and pumpkin pie spice. Transfer the mixture to the mug. Microwave on high for 2 minutes, then up to 3 minutes more in 30 second intervals (see note), until a knife inserted into the mug comes out clean. The pie will set as it cools. Use an oven mitt to remove mug from the microwave. Allow to rest at room temperature for 2 minutes to cool. Serve topped with whipped cream, and enjoy.