Preheat oven to 350 degrees and line a muffin pan with natural brown (or white) cupcake liners. In a large bowl, mix together eggs, oil and water. Add dry brownie mix and stir until smooth and thick.
Scoop batter into cupcake liners, filling each about 2/3 full. Bake 13-15 minutes, until a toothpick inserted in the center comes out clean. Remove from oven and cool completely on a wire rack.
In a medium sized bowl, use an electric hand blender to mix the butter and yogurt until smooth. Add in dry frosting mix and blend a minute more until smooth and creamy. Place four tablespoons of frosting into a small bowl and blend in 2-3 drops of black or brown food coloring. Mix thoroughly until color is completely incorporated.
Frost each brownie cupcake with the white frosting, smoothing the top of each. Place black frosting into a piping bag or a plastic storage bag. Cut off the tip of the piping bag, then pipe the Jack Skellington face onto frosting cupcakes. Serve immediately or refrigerate until ready to serve.