Preheat the oven to 160C/140C Fan/Gas 3. Grease a 20cm/8in round, deep-sided, loose-bottomed tin with butter and line the base with baking paper.
Put the cake ingredients into a large bowl and beat together until well incorporated. Spoon half the mixture into the prepared tin and level the surface.
Roll one-third of the marzipan to the same size as the base of the tin and place on top of the cake mixture.
Spoon the remaining cake mixture on top of the marzipan and level the surface. Bake for 1¾-2 hours or until golden-brown on top and firm in the middle. If the cake is beginning to brown but not cooked through cover it with tin foil. Leave the cake to cool for 10 minutes before removing from the tin.
Heat the apricot jam in a pan, then brush the top of the cooled cake with a little warm jam. Roll out half of the remaining marzipan to fit the top of the cake. Place the marzipan on the cake and use your thumb to crimp around the edges .
Make 11 balls from the remaining marzipan and place these around the edge of the cake fixing them to the marzipan with a little beaten egg.
Brush the marzipan with the egg and glaze under a hot grill for about 5 minutes (turning the cake round to ensure even browning). You can do this using a cook’s blowtorch if you prefer.