Line a 20cm cake tin with baking parchment. Melt the butter, sugar, syrup and salt together in a saucepan set over a medium heat. When the sugar has dissolved, whisk well to combine, then stir in the cereal until well coated.
Heat the oven to 180C/160C fan/gas 4. Tip the mixture into the prepared tin and use the back of a spoon to firmly press into a flat layer. Bake for 15 mins until golden and toasted. Press down again with a spoon while still warm, then leave to cool completely in the tin.
Remove the crispy cake from the tin and cut into six wedges, in the same way as cutting a pizza. Use a little icing or melted chocolate to stick on pretzels for antlers and the eye decorations on the triangles. Finish each with a red sweet for the nose. Leave in a cool place to set for 30 mins before serving. Will keep in an airtight container for up to three days.