Preheat the oven to 170°C/gas mark 3. Line a 30cm loaf tin with baking paper
Beat together the butter and sugar until pale and fluffy then add in the eggs until fully incorporated.
Gently fold in the flour then place separate the mix into a two bowls. In one bowl add the vanilla extract and mix then in the other bowl add the cocoa powder and mix.
Spoon in a of the vanilla batter into the lined tin and spread out evenly. Pour ½ of the chocolate mixture on top in a swirling motion then cover with the remaining vanilla mixture. then use a metal skewer to swirl the chocolate through the vanilla to create the marbled effect
Bake for 45–50 minutes, until a metal skewer inserted into the middle of the cake comes out clean.
Buttercream
Beat the butter until pale and fluffy then sieve in the icing sugar a bit at a time until fully incorporated.
Half the buttercream once it is fully incorporated, in one half add the cocoa powder and in the other half add the vanilla. Mix until fully incorporated.*
Add buttercreams to piping bags with star nozzles and pipe one line of chocolate buttercream then a line of vanilla buttercream until the whole cake is decorated.
Add the smarties and smarties buttons until your happy then enjoy.
Notes
*If your buttercreams are too stiff then add some whole milk until you get a soft but pliable buttercream.