Preheat the oven to 160C/325F/Gas 3. Grease and line a 20cm/8in square tin with two strips of parchment paper.
For the sponge, beat together all the sponge ingredients until smooth. Tip the cake mixture into the tin and tap lightly to level out.
Bake for about 40 minutes, or until a metal skewer inserted in the middle comes out clean.
Allow to cool in the tin for 10 minutes, then turn out and allow to cool completely on a wire rack, before putting in the fridge to chill (or alternatively place in the freezer for a few minutes until chilled but not frozen).
While the cake is chilling, make the buttercream. Beat together the softened butter and icing sugar in a bowl until lighter in colour, and smooth.
Place 100g/3½oz of the buttercream in a piping bag and allow to slightly firm up in the fridge. Keep the rest in a bowl for the cake sides.
For the marzipan topping, heat the apricot jam in a small saucepan and sieve it into a bowl.
Brush the top of the cake with the sieved apricot jam.
Roll the marzipan out very thinly, cover the top of the cake and chill again.
Cut the cake into 25 equal squares (each 4cm/1½in square). You may need to cut off the edges if they have rounded and pulled away from the sides of the tin - all the edges must be straight and neat.
Cover four sides of each square with buttercream (not the marzipan top or the base). Using the buttercream in the piping bag, pipe a blob in the centre of each square on top of the marzipan. Leave to set in the fridge for 20 minutes.
For the icing and decoration, cut the fondant icing into small cubes. Place in a sturdy free-standing mixer with a paddle. Churn the icing until it stars to break down, adding a splash of water if it's too hard. Very gradually add the water - the icing will become smooth and more liquid.
Add flavouring and food colouring to taste - be careful not to add too much at once, you can always add more but can’t undo it!
Melt the chocolate either in the microwave or in a heatproof bowl set over a pot of simmering water (do not allow the bottom of the bowl to touch the water). Once melted, place the chocolate in piping bag and set aside.
Take the cakes out of the fridge and place one onto a fork.
Dip each square into the icing one at a time and carefully set onto a cooling rack, with parchment underneath to catch the drips. Try not to get finger prints on them - for this reason it is best to insert the fork at an angle so that you can slide the cake off onto the cooling rack easily.
Leave the fondant to set, but do not put in the fridge as the icing will lose its shine.
Using the piping bag of melted chocolate, drizzle the chocolate over each fancy in a zig-zag pattern.
Leave to set and then place on a cake stand to serve.