Step 1: Preheat your oven to 350°F (175°C). Grease and flour (or use gluten-free flour) two 8-inch round cake pans, or line with parchment paper.
Step 2: In a large bowl, whisk together the flour (or gluten-free blend), cocoa powder, and baking soda. If using a gluten-free blend without xanthan gum, add 1/2 teaspoon now.
Step 3: In a separate medium bowl, whisk together the sugar (or coconut sugar), eggs (or flax eggs), milk (or plant-based milk), oil (or applesauce), and vanilla extract until well combined.
Step 4: Pour the wet ingredients into the dry ingredients. Mix until just combined. Be careful not to overmix, as this can lead to a tough cake.
Step 5: Divide the batter evenly between the two prepared cake pans. Bake for 30-35 minutes, or until a wooden skewer inserted into the center comes out clean. Let the cakes cool in the pans for 10 minutes before inverting them onto a wire rack to cool completely.
Step 6: While the cakes cool, prepare the icing. In a large bowl, beat the powdered sugar (or Splenda), softened butter (or margarine/coconut oil), and milk (or water/plant-based milk) with an electric mixer until smooth and creamy. If the icing is too thick, add more milk a teaspoon at a time. If too thin, add more powdered sugar.
Step 7: Once the cakes are completely cool, level the tops with a serrated knife if needed. Place one cake layer on a serving plate or cake stand. Spread a layer of icing evenly over the top.
Step 8: Place the second cake layer on top. Frost the entire cake with the remaining icing. If desired, divide the icing into separate bowls and add food coloring to create different monster colors.
Step 9: Decorate your Monster Halloween Cake as desired! Use a piping bag to create fuzzy monster fur, add candy eyeballs, sprinkles, or other spooky decorations to bring your monster to life. Serve and enjoy your frightfully delicious creation!
Step 10: Store any leftover cake in an airtight container at room temperature for up to 3 days, or in the refrigerator for up to 5 days.