Preheat the oven to 180°C, fan 160°C, gas 4. Grease and line 2 x 23cm cake tins.
Put the butter and sugar in a large bowl and, using an electric hand whisk, beat until light and fluffy.
Keeping the whisk running, add in the beaten egg, a little at a time, making sure the egg is fully combined after each addition.
When all the egg is incorporated, mix in the vanilla extract and sift over the flour. Carefully fold the flour into the cake mixture until you have a smooth mixture with no lumps.
Divide the mixture between the two tins and bake for 30 minutes or until the cakes are golden and risen, and spring back to the touch. Turn out onto a wire rack and allow to completely cool.
To make the buttercream icing, beat the butter and icing sugar together until fluffy. Sandwich the two cakes together using about one-third of the buttercream, then spread the rest over the top and sides of the cake. Transfer the cake to a board.
Next, make the yellow blanket. Knead the yellow food colouring into the ready to roll white icing, one drop at a time, until the icing turns an even, pale yellow colour. Dust a worktop with a little icing sugar and roll out the icing into a circle that’s big enough to cover the cake. Drape the icing over the rolling pin, then unroll it over the cake. Using your palms, smooth out the icing on the top of the cake then gently shape a ‘skirt’ around the sides with your fingers.
Decorate the Battenberg cakes to make the baby blocks. Dust a worktop with a little icing sugar and roll out half of each of the coloured ready to roll icing to a thickness of about 2mm. Using a sharp knife, cut 5 x 5mm wide strips from each and set aside – these will form the border of your blocks. Next, use cookie cutters to stamp out letters to spell out your message, and shapes to decorate the blocks, from the rolled out icing – if you don’t have cutters, you could use a sharp knife. Use the white chocolate writing icing (from the pack of chocolate and caramel writing icing) to adhere the borders, letters and shapes to the Battenberg halves.
Build the cake. Trim the skewers to between 20-25cm and insert them into opposite sides of the cake. Build your block towers by pushing the blocks onto the skewers to secure into place and spell out your message – place the ‘A’ block between the two towers, securing it with a little of the writing icing.
Using the remaining coloured ready to roll icing, make squares of about 45g each, then cut out additional shapes of your choice. Press the shapes onto the blocks, adhering with the white chocolate writing icing. Shape little bears or ducklings out of any leftover ready to roll icing and use them, along with the blocks, to decorate the cake and board.