Preheat the oven to 200C/gas 6. Whisk the egg and sugar with an electric hand whisk until thick and creamy, then stir in the flour.
Grease a 12-hole jam tart tin and put 1 tablespoon of mixture in each hole. Bake in the oven for about 10 minutes, until golden brown, then remove to a wire rack to cool.
Once cool, cut the cakes in half horizontally, so you have 2 thin cakes.
Gently heat the marmalade in a saucepan for a few minutes, until it has thickened but is still spreadable. Sift to remove any peel, if you want a smooth centre. Allow to cool, then spoon a dollop of marmalade onto the centre of each cake.
Melt the chocolate with the orange zest, oil (if you like) and water in a heatproof bowl over a pan of boiling water, stirring well. Cool until the chocolate starts to thicken, and spoon over the marmalade. Leave to set.