First, put the frozen raspberries in a saucepan with 3 tablespoons of water and the sugar. Bring to a boil and stir regularly with a whisk until a sauce is formed.
Mix the starch with a dash of water, pour it into the boiling raspberry mixture and cook for about 3 minutes, stirring constantly, to form a thick pudding – it should stick to the whisk.
If the pudding is too thin, whisk in a little more dissolved starch. Remove from heat and let cool.
In the meantime, whip the patisserie cream with the vanilla sugar and a good dash of liqueur (a berry liqueur works best, but rum or kirsch can also be used) until very stiff, about 4 minutes.
Fold in the cooled raspberry pudding and immediately pipe the raspberry cream onto the cupcakes.