Preheat the oven to 325°F. Grease the skull cake pan with cooking spray.
In a large bowl, whisk together the cake mix, eggs, water, vegetable oil and sour cream.
Fill six of the skull molds 3/4 full. Do not overfill the molds or the cakes will overflow. If you have a second skull pan, divide the remaining batter between two additional molds. Gently tap the pan(s) on the counter several times to release any air bubbles.
Bake the cakes until a toothpick inserted comes out clean, 27 to 30 minutes. Let the cakes cool for 5 minutes in the pan before inverting them onto a wire rack to cool completely. If using one pan, let it cool completely, then grease two of the molds again, divide the remaining batter between them and repeat the baking and cooling process.
Add a dollop of the raspberry or strawberry sauce to the center of each serving plate, then carefully smack a spoon down on top of it to create a splattered effect.
Place each cake on top of the sauce, then dust it with powdered sugar and serve.