Line a large tray with non-stick paper
Place a pan on the stove and half fill with boiling water.
Put the butter and marshmallows into a heatproof bowl and set it over the boiling water.
Bring the water back to a simmer.
Let the butter melt and once it has stir occasionally until the marshmallows are completely melted and smooth (see note 3)
Add the salt and stir to combine.
Turn the heat off from under the pan and add the rice krispies to the melted mixture.
Gently stir until the mixture is well combined. It will seem like this will never come together but eventually it will all be coated.
Lightly oil two tablespoons (front and back) and use them to scoop and shape a mound, set the mound down onto your prepared tray and with lightly oiled fingers flatten to a rough brain shape.
Repeat until you have 5 'brains'
Lightly coat a table knife with oil and use it to press the central indent into your brain. Use your oiled fingers to hold the oval shape together.
Continue forming 5 brains and then doing the indents until you have used all the mixture.
Use the red icing to add spots or drizzles of blood to your brains.
Leave the brains to set for 4-5 hours.