Begin by sifting the flour with the baking powder in a bowl and then put to one side. Put the sugar and the water in another bowl and mix well with a whisk until the sugar dissolves. Cut lengthwise along the vanilla pod with a knife and scoop out the seeds, then add them to the water and sugar; pour in the oil and the lemon juice too and mix thoroughly. Grate the lemon rind into the mixture as well. Now take the flour and baking powder and add them, one spoonful at a time, to the rest of the ingredients. Stir well, constantly, in order to avoid lumps forming. You will obtain a smooth, soft mixture. Take the cake tin, grease it with a tablespoon or two of oil, “spread” with kitchen paper and dust it with 2 tablespoons of flour, sprinkled through a sieve. Pour in the mixture distributing it evenly and then bake it in the oven, preheated to 180 °C, for approximately 50 minutes. If the cake “browns” too much while cooking, after at least twenty minutes (otherwise it will deflate), turn the temperature down slightly and cover it quickly with a sheet of kitchen foil. Before removing it from the oven do the “toothpick test”: stick one into the cake, if it comes out dry it’s cooked. Take it out of the oven and leave it to cool down; then open the spring and slide the cake delicately onto a plate.
Decorate it with a dusting of icing sugar (sprinkled through a sieve) and with 2-3 spoonfuls of Fragolina (or jam, or melted chocolate, or pastry cream, according to your taste and imagination).
Notes: it keeps for 2-3 days at room temperature, better still if covered with a glass dome. If you like you can freeze it: Once defrosted (in the fridge, “naturally”) it will be as delicious as if just baked. The cake can be filled with Fragolina, pastry cream, chocolate, jam or whatever you prefer (the same sweet “toppings” used to decorate it) after cutting it into two disks with a sharp knife.