Preheat the oven to 180°C, fan 160°C, gas 4. Grease a 20cm square cake tin and a 450g loaf tin and line both with baking parchment. In a large bowl, mix together the flour, baking powder and caster sugar.
Stir in the cocoa/water and the egg/water mixtures. Add the sunflower oil and melted butter, then stir well until combined. Half-fill the loaf tin with the mixture and bake for 30-35 minutes. Pour the rest into the square tin and bake for 60-65 minutes. The cakes will be ready when a skewer inserted into them comes out clean. Cool on a wire rack.
For the icing, use an electric hand mixer to cream the butter in a bowl. Add the icing sugar, spoon by spoon. Mix the cocoa, vanilla extract and 2-3 tablespoons boiling water to make a thick paste, then beat into the buttercream.
Trim the cakes to create an even surface. Trim the edges of the loaf to make a brick shape, then cut in half vertically. Use some of the buttercream to sandwich the halves together vertically to make a tower.
Halve the square cake horizontally and sandwich with some more buttercream. Using a palette knife, coat the large cake in buttercream. Place the smaller cake in the centre on top and coat smoothly in buttercream.
Stick the chocolate biscuits side by side all around the top tower. Place biscuits at each corner of the base to create turrets, then cut the remaining biscuits to the height of the cake and place around the base.
Tip the sugar-coated chocolate buttons onto the tower and around its base. Place the candles around the fort to serve.
Cook's tip: mix and match dark and milk chocolate finger biscuits to give your castle a rustic finish.