Melt the Ghirardelli white melts according to package directions. Fill the skull mold with the white chocolate, tap gently to remove air bubbles, and refrigerate until firm.
Preheat the oven to 350°F. Spray three 6-inch pans with baking spray.
In a large bowl, mix together the cake mix, oil, melted butter, milk, eggs plus yolk, sour cream, and vanilla until smooth.
Divide the batter evenly among the pans, smooth the tops, and bake 24–26 minutes, or until a toothpick comes out with a few moist crumbs. Cool in the pans for 10 minutes, then transfer to wire racks.
Beat the butter until creamy. Add the powdered sugar, cocoa, vanilla, and heavy cream. Mix until fluffy and spreadable.
Place one cake layer on a plate or board and spread with a thin layer of frosting. Add the second layer and repeat. Top with the final layer.
Cover the cake with a thin crumb coat of frosting. Refrigerate 15–30 minutes, then frost with a smooth final layer. While frosting is still soft, press white chocolate skulls around the base.
Warm the chocolate drip as directed and pour over the top, letting it drip slightly down the sides.
Arrange 8 Halloween Oreos evenly around the top. Pipe chocolate frosting swirls between them using a 1M piping tip. Fill the center with the white sprinkles.