Preheat your oven to 140C Fan/325F/Gas Mark 3. Line a cupcake tin with cupcake cases, preferably black.
In a mixing bowl, combine the softened baking spread and caster sugar using an electric mixer until light and fluffy. Add in the eggs one at a time along with the vanilla extract, whisking until well incorporated.
Gently fold in the self raising flour until just combined. Add the black food coloring gradually until you achieve a deep color.
Divide your batter evenly among the lined cupcake cases and bake for 25-30 minutes or until a skewer inserted into the center comes out clean. Transfer to a cooling rack to cool completely.
Once cooled, use a cupcake corer or knife to remove a small section from the center of each cupcake. Mix the tinned custard with green food coloring and spoon it into each hollowed cupcake.
Mix together softened butter and icing sugar until smooth. Add vanilla extract, milk, and both green and yellow food coloring. Pipe or spread this buttercream onto each filled cupcake and decorate with candy eyes and green sprinkles.