Preheat the oven to fan 140C/ conventional 160C/gas 3. Grease the sides of a Swiss roll tin, about 32cm x 23cm x 2cm, with butter. Place the tin on a sheet of non-stick baking parchment and draw round it with a pencil. Cut along this line so you are left with a piece of parchment the same size as the tin base. Press it into the tin.
Put the butter and syrup into a small saucepan. (If possible, place the pan on some scales first and note the weight before pouring in the syrup). Gently warm the pan over a low heat until the butter has completely melted, then stir well to combine. Remove from the heat and cool for about 15 minutes.
Tip the flour into a large bowl with 1 tsp salt. Stir in the muscovado sugar with a wooden spoon. Beat in the cooled syrup and butter until mixed. If you find this hard to do, ask an adult to help.Crack the eggs into a jug. Pour in the milk and beat with a fork. Now pour this into the bowl with the flour and sugar and stir until the mixture is smooth.
Pour the mixture into the tin and tap gently on the work surface to level it. Put it on a baking sheet larger than the tin. Bake for 40-50 minutes until risen and firm in the centre. Check the cake is ready by getting an adult to press it with their finger. It should be firm and springy.
Ask an adult to remove the tin from the oven, then leave it to cool for 10 minutes. Use a round-bladed knife to trim any cake that has come over the sides. Run the knife inside the sides of the cake tin, tip the cake onto a wire cooling rack and leave for 2 hours or until cold. Then slide the cake onto a board and trim off the four edges so you have a 30cm x 20cm rectangle. Cut lengthways into 5cm wide strips.
Mix half the icing sugar in a cup with a drop of warm water to a thick paste, using a teaspoon. Set aside. Sift a little icing sugar onto a worktop and divide the marzipan into three. Roll each piece into a ball. Make a dent with your thumb in one of the balls and, using a cocktail stick, drip in a few drops of red colouring. Knead the ball until evenly coloured. Repeat with a second ball and green colouring. Leave the third ball plain. If you don’t want to get the colour on your hands,wear clean kitchen gloves.
Dust some more icing sugar onto the worktop and roll out the balls to the thickness of a £1 coin. Using a small star cutter (or try other shapes, such as holly), stamp out eight stars in each colour. Dab a blob of icing on each shape and stick six along each cake strip. Cut between them to make 24 squares.
Dust the top of the cakes with the rest of the icing sugar. Arrange on paper plates, covered in cling film, or in pretty boxes.