Heat the oven to 180C (160C fan)/350F/gas 4. Grease a 20cm cake tin and line it with baking paper. Cut one orange into thin rounds and lay these neatly in a single layer over the base of the prepared tin.
Cream the butter and sugar in a food mixer. Mix in the beaten egg a third at a time, and stir in a spoonful of the flour if it threatens to curdle.
Sift in the flour and ground almonds, mix until smooth, then grate in the zest of the remaining orange and add a squeeze of its juice (keep the rest of the juice for the drizzle). Stir in the rosemary and marmalade.
Spoon the mixture into the tin, being careful not to dislodge the orange slices on the base, then bake for 30-45 minutes, until a skewer inserted into the middle comes out clean. Remove from the oven and leave the cake to cool in its tin for 15 minutes.
Meanwhile, make the drizzle. In a small pan, warm the remaining orange juice, sugar and water over a medium heat, and stir until the liquid turns into a syrup.
Turn the cake out on to a plate, prick holes all over the top with a skewer and pour over the syrup. Leave for a few minutes for the syrup to sink in, then serve warm as a pudding, with cream or ice-cream. Once cold, eat as cake.