Preheat your oven to 140C Fan/325F/Gas Mark 3 while preparing your cupcake tin with black cupcake cases.
In a mixing bowl, combine the baking spread and caster sugar using an electric mixer until smooth and fluffy, about five minutes.
Add the eggs one at a time along with the vanilla extract, whisking them in well to ensure everything is nicely combined.
Gently fold in the self-raising flour followed by the black food coloring until you achieve a uniform dark batter.
Divide the batter into your prepared cupcake cases and bake for 25-30 minutes, ensuring they are cooked through by testing with a skewer before cooling on a rack.
Once cooled, core out each cupcake and fill them with lime-colored custard before topping them off with bright green buttercream made from icing sugar, butter, vanilla extract, milk, and food coloring; finish with candy eyes and sprinkles.