Preheat the oven to 350 degrees and grease a 9x13 baking dish with nonstick cooking spray.
Melt 1/2 cup of butter and combine it with the sweet potatoes, milk, brown sugar, vanilla, ground cinnamon, nutmeg, and ground ginger in a medium bowl. Mash the ingredients into the potatoes until smooth.
Spread the sweet potato mixture in the bottom of the prepared pan.
Sprinkle the cake mix over the top of the sweet potatoes.
Cut the remaining 1/2 butter into slices and arrange the slices evenly over the top of the cake mix.
Bake for 45 to 55 minutes, until the top is golden brown. Allow to rest for 15 minutes before slicing and serving.