Preheat oven to 180°C ( 356°F )
Remove the pineapple slices from the jar, drain in a colander and place on kitchen paper to dry thoroughly.
Place the pineapple slices flat on the caramel in a circular pattern, with one slice in the center. Place a candied cherry in each hole and set aside.
Melt the butter and leave to cool.
In a bowl, whisk the eggs with the sugar and vanilla sugar until frothy. No need to blanch or triple the volume.
Stir in the warm melted butter and the pineapple syrup.
Add the sifted baking powder, followed by the flour, and mix well.
Add a few pineapple slices, cut into pieces, to the batter (optional).
Pour into the pan and bake for 45 to 55 minutes, more or less. Check the doneness after 45 minutes by inserting a wooden skewer into the cake. It should come out dry.
Tip: You can butter the sides of the round cake pan to make unmoulding easier. I don't butter the sides of my cake pan, and the cake comes out perfectly. You can get a taller cake by using a smaller pan. I find that this way, the cake gives nice slice that are neither too thin nor too thick.
Soak the cake with a few spoons of syrup after removing it from the oven (optional), then return it to the turned-off oven for 5 minutes.
Turn out onto a serving dish and leave to cool or just lukewarm before serving.