Soak the gelatine leaves in cold water according to the packet's instructions (I soak mine for 8-10 minutes).
Prepare strawberries: Clean the strawberries. Chop about ⅓ of the strawberries and set them aside. Keep about 2 or 3 for decorating purposes, if you wish. Puree the rest of the strawberries together with the sugar.
Dissolve gelatine: Squeeze the gelatin gently to remove excess water. Place the gelatin leaves in a saucepan and dissolve over very low heat. Do not overheat the gelatin, and do not let it start to boil. Dissolving the gelatin will take a very short time, not even a minute, so stay close and keep stirring.
Add strawberry puree: Take the saucepan off the heat and start mixing in the strawberry puree a very little at a time. Start by adding one tablespoon puree, incorporate it well, then add another tablespoon. After adding about ⅓ of the puree, you can start adding more than just one tablespoon at a time but keep stirring well and continuously.
Refrigerate: Place in the fridge for about 30 minutes or until just starting to set (it might take less time than that, depending on your fridge, so start checking after 5-10 minutes already, the mixture should not set too much).
Whipped cream: Whip 375 ml/ about 1 ⅔ cup of the heavy cream and vanilla sugar. Fill a little bit of it in a piping bag and pipe an edge on the bottom of an 18 cm/7-inch springform. Take the strawberry puree out of the fridge and fold in the chopped strawberries you set aside before and the whipped cream.
Assemble: Place the ladyfingers with the sugared side on the outside inside the pan to form a tight ring around the springform base. Fill the mousse into the prepared form.
Refrigerate for at least 4 hours or until the strawberry mousse is set.
Decorate: Whip the remaining heavy cream and decorate the cake. Slice the leftover strawberries and use them to decorate the charlotte as well.