Begin by preheating your oven to 350 degrees F. Butter and flour your skull cake pan and set aside.
In a small sauce pan, heat the butter, cocoa powder, salt, and water over medium heat. Cook and stir just until melted and evenly combined. Remove from heat and set aside.
In a large bowl, whisk together the flour, sugar, and baking soda to combine. Add half of the melted butter mixture and whisk until blended. The mixture will be thick. Add the remaining butter mixture and whisk to combine. Add the eggs, one at a time, whisking in between until completely blended. Finally, mix in the sour cream and vanilla until smooth.
Add the batter to the prepared pan, filling each side to about 90% full. (You may have a little extra batter.) Bake until a wood skewer inserted near the center comes out clean (45 to 50 minutes). Let the cake cool completely in the pan.
While the cake cools, prepare the chocolate glaze. To a large bowl, add the chocolate and corn syrup. Set aside. In a small sauce pan, combine the heavy cream and granulated sugar. Stir over medium heat until the cream is hot and the sugar is dissolved. Pour the hot cream over the chocolate and whisk until smooth.
In a small bowl, whisk together 1 cup of the prepared chocolate glaze with 1/2 cup confectioners' sugar until combined.
In another bowl, prepare the powdered sugar glaze by whisking together the confectioners' sugar, milk, and vanilla until smooth. For a thinner glaze, use additional milk.
Once the cake is cooled, trim off any cake that has domed above the top of the pan with a serrated knife to create a flat surface on each piece of cake. Use the chocolate glaze mixed with the confectioners sugar to sandwich the two cake halves together. Fill in any cracks with extra glaze. Refrigerate for about 30 minutes.
Place the cake on a baking rack that's on top of a baking sheet. Use a pasty brush to apply the powdered sugar glaze. Allow any excess glaze to drip off onto the baking sheet.
Move the cake to a cake stand or serving platter. Drizzle with some of the chocolate glaze. Serve the cake with any additional chocolate glaze in a bowl on the side.