Preheat oven to 180°C, fan 160°C, gas 4. Grease and base-line a 900g loaf tin.
In a large bowl, beat together the butter, sugar and orange zest until the mixture is pale and fluffy.
In a separate bowl, lightly beat the eggs with a fork, then add gradually to the mixture, stirring all the time. Sieve in the flour and baking powder and gently fold into the mixture.
Spoon the batter into the prepared cake tin and bake for 30 mins. If a skewer doesn’t come out clean, cover the cake with baking parchment and bake for another 10 mins, until risen and golden. Let cool, then lift out onto a rack.
For the icing, whisk together the orange juice, icing sugar and cocoa powder. Cut the top off the cooled cake and then spread the icing over the outside of both halves. Decorate the lid with strawberry pencils and chocolate beans. Place the chocolate coins, mixed sweets and berries on and around the bottom half, as though it’s treasure spilling out, and rest the ‘chest’ lid on top.