Put the eggs, salt, sugar and vanilla sugar in a bowl and whisk for 5-7 minutes until pale and fluffy.
Pour in the vegetable oil and lemon juice, add the zest, and mix again.
Sift in the flour with the baking powder, fold in gently. Divide the batter among the cupcake liners.
Bake for 20-25 minutes in an oven preheated to 170-180°C, check for doneness with a skewer. Let the cupcakes cool completely.
For the filling, put the currants, sugar and cornstarch dissolved in water into a saucepan, bring to a boil and cook until thickened. Remove from the heat and blend smooth.
Let the jam cool, then fill the cupcakes with it.