Preheat the oven to 350 degrees F and line an 8x8 inch pan with parchment paper.
In a medium bowl, whisk together the flour, sugar, cocoa powder, baking soda, and salt until evenly mixed.
In a separate bowl, combine the buttermilk, oil, egg, vinegar, vanilla extract, and green food coloring. Stir until the color is uniform.
Pour the wet mixture into the dry ingredients and stir gently until just combined and no dry streaks remain.
Spread the batter evenly in the prepared pan and bake for 22 to 25 minutes, or until a toothpick inserted in the center comes out clean.
Let the cake cool completely in the pan before frosting.
Beat the cream cheese and butter together until smooth, then add the powdered sugar and mix until creamy.
Spread the frosting over the cooled cake and top with candy monster eyes and green sprinkles.