Grease and line a 21cm (8-inch) square tin, about 5cm (2-inches) deep with nonstick baking parchment. In a large pan, melt the vegan margarine. Add the vegan marshmallows and vanilla and stir gently over a low heat until the marshmallows have completely melted and the mixture is smooth. It will feel like it won’t come together but be patient, it will. Switch the heat off.
Fold in the rice krispies and mix with a silicone spatula until they are all completely coated.
Turn out into the lined tin and using the back of a silicone spatula (grease it with some vegan margarine if you need to), smooth the top to make it flat. Press it gently to compact it slightly but not so much that you crush the cereal. Scatter your favourite sprinkles over the top if you’re feeling fancy.
Allow to cool at room temperature and then cut into squares as big or as small as your heart desires.