Boil sugar and water for 5 minutes and then mix with strawberries. Set aside one third of this strawberry sauce.
Whip the cream with cream stiffener until firm. Mix the mascarpone with the strawberry sauce and the vanilla sugar and fold in the whipped cream.
Soak the tiramisu biscuits in the strawberry sauce set aside and line the bottom of a mold with them. Alternate layers of mascarpone cream and biscotti, finishing with the mascarpone cream.
Refrigerate for a few hours and sprinkle with cocoa before serving.