Line a 20cm square cake tin with baking parchment. Tip the butter, dark chocolate and marshmallows into a saucepan over a low heat and warm for 2-3 mins, stirring occasionally until melted together. Watch closely to ensure the mixture doesn’t catch. Remove from the heat and leave to cool for a few minutes.

Tip the cereal into a large bowl and pour over the warm melted chocolate mixture. Stir well with a silicone spatula until all the cereal is coated. Tip the mixture into the prepared tin and use the back of a spoon to press it firmly into the base and sides. Put in the fridge while you prepare the caramel layer.

For the caramel, pour the condensed milk into a deep saucepan along with the butter and sugar. Warm over a low heat, stirring with a spatula until the butter has melted. Turn the heat up to medium and simmer for 5-10 mins until the mixture thickens and turns golden – when it’s ready, a line should form when you run your spatula along the base. (Don’t worry if there are some darker specks, just keep stirring the caramel so it doesn’t burn.) Pour the caramel over the cereal base and smooth the surface. Chill for at least 1 hr until set.

Melt the white chocolate in a heatproof bowl in the microwave in 5-second blasts (about 1-2 mins total), stirring halfway through. Or, do this over a pan of simmering water, ensuring the base of the bowl doesn’t touch the water. Divide the melted chocolate between five bowls. Leave one bowl plain, then colour the remaining four bowls with a few drops of food colouring each. (We chose pale yellow, blue, green and pink.) If the chocolate starts to set as you’re mixing in the colours, return it to the microwave for about 30 seconds.

Alternately spoon dollops of the coloured and plain white chocolate over the set caramel layer. Shake the tin gently until the surface is smooth. Use a skewer to gently swirl the chocolate to create a marbled effect, then scatter over the chocolate eggs and chill for 1 hr until set. Remove from the tin and cut into nine larger or 16 smaller squares.