Put the margarine, oil, caster sugar, milk, cocoa, eggs and self-raising flour in a large mixing bowl.
Whisk, starting slowly and then speeding up until you have a light, smooth batter.
Line a 12-hole muffin tray with cupcake cases and spoon the batter evenly between them. I find the easiest way to do this is with a large ice cream scoop. Don't forget to level off.
Bake for 20-25 minutes until springy to the touch. If you take them out too soon, they'll sink.
Transfer the cakes to a wire cooling rack and allow to cool completely.
Make the slime
Put the caramel into a bowl.
Add the green food colouring. You may need more or less depending on how concentrated your colouring it.
Mix until you have a nicely spooky green colour.
Fill the cakes
Use an apple corer to make holes in the cakes to about half way down. You can eat the offcuts!
Fill the holes with the caramel, with more caramel spilling on top of the cakes.
You can do this by transferring the slime to a piping bag and piping it into the cakes as I did, or you can simply use a spoon.
Your cupcakes are now ready to enjoy!
Place the cupcakes in a sealed container in a single layer. Don't stack them as they will stick together.You should place the container in a cool, dry place, such as a pantry, or in the refrigerator.Stored correctly, your slime cupcakes should keep for at least three days. However, they're at their best when enjoyed within 24 hours of making.