Set the cake on a cake board or cutting board. In a small saucepan, warm the jam over medium heat until fluid, then pass through a fine-mesh sieve into a bowl. Using a pastry brush, brush the warm jam over the top and sides of the cake.
Lightly dust a work surface with sugar and place the marzipan on it. Knead until soft and pliable, then shape into a thick disk. Lightly dust a rolling