Preheat an oven to 180 degrees/170 degrees fan/gas mark 4.
Grease and line a baking tin or roasting tin approx. 30cmx20cm
Cream together the softened butter and sugar with an electric whisk or mixer until light and fluffy.
In a separate bowl, beat together the eggs, milk and Vanilla Extract and add to the butter and sugar.
Add the flour, Baking Powder and Cocoa Powder to the mixture and mix with an electric whisk or mixer until smooth.
Mix the Jumbo Chips and White Chocolate Chunks in a little flour to coat them then fold through the cake mixture. Coating the chocolate chunks/chips in flour prevents them from sinking to the bottom enabling the chocolate to be dispersed evenly through the batter.
Pour the mixture into your prepared tin and spread evenly.
Bake in the preheated oven for 35-40 mins until a skewer inserted comes out clean.
Cool in the tin for 15 minutes and then remove from the tin and cool on a wire rack.
Once cooled, spread the top of the traybake with the Chocolate Buttercream Style Icing using a palette knife.
Use sprinkles to decorate your traybake, and Letters and Numbers to personalise your bake.