To make the marble chocolate, place a piece of baking parchment on a baking tray. Place the dark chocolate in a heatproof bowl and melt over a saucepan of simmering water. Do not allow the water to touch the bottom of the bowl. Pour into a pool of chocolate about 12cmx20cm over the baking parchment. Repeat the above chocolate melting process with the white chocolate. Place the white chocolate in a piping bag. Snip the end off and pipe zigzags over the dark chocolate. Use a cocktail stick to create a feathered pattern look. Leave to fully dry.
To make the cupcakes, preheat the oven to 180C/Fan160C/Gas Mark 4. Place 12 muffin paper cake cases in a 12-hole muffin tin.
Place the butter and sugar in a large mixing bowl. Beat with an electric hand held beater until light and fluffy. Slowly add the eggs beating well between each addition, adding a spoonful of flour if the mixture starts to curdle. Fold in the flour followed by the orange zest and juice using a large metal spoon. Stir in the orange gel food colouring, to your colour liking.
Divide the batter equally between the 12 paper cases. Bake in the centre of the oven for 20-25 minutes or until risen and firm on top.
Allow the cakes to cool then place on a cooling rack.
Meanwhile, to make the butter icing, place the butter, vanilla extract, milk and half the icing sugar in a large bowl and beat with a large metal spoon until smooth. Mix in the remaining icing sugar.
Fill a piping bag fitted with a 1cm star nozzle with the buttercream. Fold the top of the bag. Pipe swirls of icing on top of the cupcakes. Break the chocolate marble into 12 pieces and use to decorate the top of the cupcakes.