Heat the oven to 180C/160C fan/gas 4. Butter a 1kg loaf tin (ours was 14 x 24 x 7cm) and line with a long strip of baking parchment. Lightly dust a surface with icing sugar and roll the marzipan out to a 1cm-thick rectangle, then cut it so that it’s just smaller than the base of the loaf tin.
Beat the butter and both sugars together in a large bowl using an electric whisk until light and fluffy. Beat in the eggs, flour, baking powder, almond extract, milk and cocoa powder until smooth. Fold in the chocolate chips, scraping down the sides of the bowl to ensure everything is combined.
Pour half the batter into the tin. Top with the marzipan rectangle, then pour the remaining batter on top and smooth the surface. Bake for 1 hr-1 hr 15 mins until golden, risen and a skewer inserted into the middle comes out clean. Leave to cool completely in the tin.
To make the ganache, bring the cream to a simmer in a small pan. Meanwhile, put the dark chocolate, golden syrup and almond extract in a heatproof bowl. Once the cream is simmering, pour it over the chocolate mixture and set aside for 2 mins to melt. Whisk until smooth, then leave for 5 mins to cool and thicken slightly. Pour the ganache all over the cake, and cut into slices to serve.