Make the custard: blend the sugar, custard powder and cornflour with 1 tbsp milk to make a runny paste.
Beat in the egg yolk. Pour the remaining milk into a pan, scrape in the vanilla seeds, add the pod, then allow to come just to the boil. Stir this into the cornflour paste, then pour into a clean pan. Cook over a medium heat, stirring all the time, until thickened.
Pour into a bowl, cover the surface with cling film to stop a skin forming, allow to cool, then chill until completely cold.
Make the cake: heat oven to 180C/ fan 160C/gas 4. Lightly oil and line the base of a 20cm round cake tin. Put the sugar and eggs into a bowl. Whisk with electric beaters for 5 mins until very thick, paler in colour and the consistency of whipped cream. Sift over the flour and quickly, but lightly, fold it in. Spoon the mixture into the tin and carefully level it, being careful not to squash it. Bake for 25 mins until risen, then remove and cool on a wire rack. Peel off lining paper.
Halve the cake so you have a semi-circle. (The other half can be frozen for another time.) Split the semi-circle in half with a knife, then sandwich back together with fruit spread.
Put the sugar and lime zest for the fruit into a pan with 2 tbsp water. Bring slowly to the boil until the sugar has dissolved, then bubble for 1½-2 mins until syrupy. Tip in the blueberries, then cook very briefly, stirring once or twice only, just until they start to burst and release their juices (but still stay whole) and you get a purple syrup. Set aside to cool.
The sponge and custard layers can be built up to 2-3 hrs ahead of when you want to serve. Cut the jammy sponge into cubes, then place in the base of a glass dish. Drizzle over the Marsala. Keep about ¼ of the berries back for the top, then spoon the rest over the sponge with a little syrup. Discard the pod from the custard, then pour it over the fruit.
Just before serving, beat the yogurt, mascarpone and sugar together until smooth and creamy. Pile onto the custard, then drizzle over the reserved fruit and syrup. Use a skewer to swirl some of the juices through the creamy topping. Serve straight away or the syrup discolours the topping. This is best eaten the same day.