Pre-heat your oven to 140C Fan/325F/Gas Mark 3, and line a cupcake tin with cupcake cases (ideally black ones).
For the cupcake sponge, mix together the baking spread and caster sugar, ideally using an electric mixer, for 5 minutes until smooth and fluffy.
Add the eggs and vanilla extract and whisk them in well until fully combined and smooth in texture.
Then gently whisk in the self-raising flour until just combined to maintain fluffiness in your batter.
Add the black food coloring and mix in well until you achieve a uniform dark color throughout your batter.
Divide the mixture between the cupcake cases evenly, then bake them for 25-30 minutes or until a skewer inserted in the center comes out clean.
Put them on a rack to cool completely before proceeding to fill them.
Once fully cooled, use a cupcake corer or knife to create holes in each cake where you will fill them later.
Add a little bit of green food coloring to tinned custard, mixing it well before spooning it into each cupcake's center.
To make the buttercream, mix together softened butter and icing sugar until smooth; then add vanilla extract, milk, green food coloring, and yellow food coloring until you reach a bright green hue.
Using a piping bag, pipe or spread your buttercream onto each cooled cupcake creatively before decorating with candy eyes and sprinkles.
Store any leftovers in an airtight container at room temperature; consume within three days for best freshness.