Preheat your oven to 350°F (175°C) and line a cupcake pan with liners. In a large mixing bowl, whisk together the flour, sugar, baking powder, and salt. In a separate bowl, combine buttermilk, water, oil, egg, and vanilla extract, mixing until smooth. Pour the wet ingredients into the dry mixture, stirring gently until just combined. Fill the liners about two-thirds full and bake for 18-20 minutes, or until a toothpick comes out clean. Let cool for 10 minutes in the pan before transferring to a wire rack.

While the cupcakes cool, prepare the vanilla buttercream frosting. In a large bowl, beat the unsalted butter on medium speed until creamy and smooth, about 2-3 minutes. Gradually add in the powdered sugar, mixing thoroughly. Incorporate the vanilla extract and a pinch of salt, adjusting the consistency with heavy whipping cream until you achieve a smooth and spreadable texture. Set aside a portion of the white frosting for crumb coating.

Once your buttercream is ready, divide it into several small bowls for coloring. Add gel food coloring to each bowl, mixing until you achieve vibrant shades for your Mini Monster Cakes. Reserve some white frosting to use as a base layer for crumb coating.

Take two cooled vanilla cupcakes and place one on top of the other to create a stack. Use a small amount of frosting between the layers to hold them together. Insert a skewer through the center of the stacked cupcakes for additional stability. Next, apply a thin layer of reserved white frosting around the outsides of the cakes to create a crumb coat. Freeze the stacked cakes for about 15-20 minutes to firm up.

Remove the crumb-coated Mini Monster Cakes from the freezer and begin decorating. Use your colored buttercream with various piping tips to create fluffy frosting all around the cupcakes, mimicking monster fur. Add fondant or colored chocolate for eyes and mouths, and sprinkle on eyeball sprinkles for extra fun.