Grease and line an 9-inch/23-cm cake tin with parchment. Heat the oven to 350°F/175°C. Whisk the eggs and sugar, preferably with an electric mixer, until tripled in volume, and thick and ribbony like whipped cream. Add the vanilla. Scatter over the flour, a spoonful at a time, and gently fold it in, without over-mixing. Pour the batter into the pan and bake until a toothpick inserted in the centre some out clean, about 25 minutes. Let cool in the pan for ten minutes, before removing to a rack and cool completely.
Prepare the topping: Melt the jelly with a tablespoon of water in a saucepan and strain. Lay the cake on a rack, bottom up, and brush the jelly mixture over the surface. Roll out the marzipan exactly to the size of the cake and lay it on top. Trim the edges of the cake, wrap, and chill for half an hour so it will cut neatly.
While the cake chills, make the icing: put the corn syrup with two tablespoons warm water in a saucepan and heat to dissolve. Beat in the sugar, adding about 2 more tablespoons of warm water (or part liqueur, if you like) to make a smooth icing which, when poured, will drape over the cakes like a satin sheet. Tint the icing with a few drops of food colouring, if you like.
Remove the cake from the refrigerator and cut into perfect 1-inch/2.5 cm squares with a sharp knife. Place them on a wire rack set on a baking sheet. Spoon the icing over each “cakelet” to coat completely. Let it set, and repeat. Decorate, and store in an air-tight container until serving.
For a more moist cake, brush a couple of tablespoons of liqueur over the cake before the jam.