1. Preheat the oven to 325 degreed Fahrenheit and grease the skull cake pan with cooking spray or melted butter. Make sure all the crevices of the cake pan are greased.
2. In a large mixing bowl, combine the cake mix, milk, butter and eggs. Mix with an electric mixer over medium speed or beat vigorously by hand until combined.
   
3. Divide the cake mixture among the skull cavities. The cavities should be filled almost to the top, with about 1-inch left from the top.
   
4. Bake in the preheated oven for 45-50 minutes, checking the cake after about 45 minutes of cooking time.The cake is ready when a cake tester or wooden skewer comes out clean when inserted in the middle of the cake.
5. Allow the cakes to cool in the pan for 20 minutes, then invert them onto the cooling rack. Cool the cakes completely before assembling.

Make the Buttercream

1. Using a handheld or stand mixer fitted with the paddle attachment, beat the butter on medium speed until creamy, about 2 minutes.
   
2. With the mixer on low, slowly add half of the powdered sugar, the cocoa powder, milk and vanilla and mix until smooth. Slowly add the remaining powdered sugar and pinch of salt. With the mixer on medium speed, mix until completely smooth, about 3 minutes. If the frosting is too stiff, add more milk. If it's too thin, add more powdered sugar.

Assemble:

1. To assemble the cake, use a serrated knife to remove the domes from the top of the cakes, if needed.
   
2. Return the front half of the cake to the pan. Spread a thin layer of buttercream on the front half (the half that is in the pan). Take the back half of the cake and press down gently onto the buttercream to securely sandwich the halves together. For additional stability, place a 4 inch skewer into the base of the skull and another one into the upper half of the skull on the back side of the cake. Cover the cake with plastic wrap and refrigerate for 30 minutes so the buttercream has time to harden. In the meantime, make the glaze.
   

Make the Glaze

1. Add the powdered sugar to a medium bowl. Slowly and gradually stir in the milk and vanilla and mix until it becomes a pourable glaze. Set aside
   

NOTE: If you want to make some faux blood, remove about 1/4 cup of the glaze and pour it into a small bowl. Then, color it with red food color gel.

Assembly Continues

1. Carefully remove the skull cake from the pan and set it on a plate (see photo). Spread some of the buttercream on the cake seams to fill in any cracks. Smooth the buttercream with an offset spatula.
   
2. Pour the glaze over the cake and allow it to set. You can decorate it with some of the red "faux blood" glaze.
   
3. To serve, you can carefully transfer the cake to a serving platter. Slice the cake from the top of the skull and then cut those slices in half. Serve.