Place the butter in a bowl and gradually beat in the icing sugar until light and fluffy. Stir in the orange rind and juice.
Cut the cake horizontally into 3 short strips. Cut each strip into 3 triangles and use the half-triangle left at each end to make a fourth triangle, butting the halves together with a little butter icing. Create a total of 12 triangles – 9 whole ones and 3 made of butted-together halves.
Spread the tops and sides of the cakes thinly with butter icing to stick the crumbs in place, then spread with a thicker layer of icing. Arrange the triangles on a cake board or plate.
Place a marshmallow at the base of each triangle for a tub. Press on sugar pearls for decorations.