Quick & Easy Rainbow Traybake

Preheat the oven to 180ºC / 160'c fan/ Gas 4. Grease a 23 x 28 cm shallow traybake tin and line the base with baking parchment paper. 

Using an electric mixer beat together the butter, caster sugar, eggs, self raising flour and baking powder for about 2 minutes until soft and smooth.

Spoon the mixture into the tin and then spread it to ensure the whole tin is filled to the same level. Bake for 35-40 minutes until well risen and the centre of the cake springs back when lightly pressed with a fingertip.

Allow it to cool for  about 10 minutes in the tin before turning it out onto a cooling rack and allowing to cool completely.

To decorate, beat together the butter, icing sugar and vanilla extract until soft and smooth. Separate the buttercream into 5 equal portions. Add a small squeeze of food colouring gel to each portion and mix thoroughly. You can choose any colours you like for this. 

For each piping bag, use a large nozzle- it doesn't matter which shape they are, as the applied buttercream will be smoothed afterwards. Fill your piping bags: one for each colour. Using your first bag, pipe the corner of the cake to form a triangle. You want to aim for around an inch width for each colour.Use the piped triangle as a guide for where to start your next colour. Using your second colour, form a large stripe next to the triangle making sure it touches. Continue to do this, alternating all your colours until your cake is covered with buttercream stripes.

Use a cake smoother to blend the stripes into an overall covering. Make sure you follow the direction of the buttercream stripes so the colours don't bleed. Rinse and dry your smoother each time to stop the colours mixing. 

Once you've finished, cut your cake into squares and serve.