Butter a 34 x 24 x 5cm deep baking tray and line with baking parchment. Melt the butter and vanilla in a large saucepan over a low heat, then add the marshmallows and cook gently, stirring occasionally until everything is melted, smooth and glossy.

Remove from the heat and mix in the rice cereal and 150g of the white chocolate chunks. Stir until the cereal is evenly coated, then tip into the baking tray and press into the corners using slightly wet hands to prevent the mixture from sticking to you. Leave to cool for 1 hr until set.

Melt the remaining 50g white chocolate in a heatproof bowl set over a pan of simmering water, ensuring the base of the bowl doesn’t touch the water. Drizzle the melted white chocolate over the crispy cakes and sprinkle over the mini marshmallows, if using. Leave to set, then cut into 12 squares. Cut the squares in half diagonally to serve. Will keep in an airtight container for up to two weeks.