Soak the gelatin sheet in a bowl with cold water for about 10 minutes.
Pour cream and milk in a saucepan, add sugar and a whole tonka been. Warm cream and milk until they begin to simmer, then remove from the heat. Let sit for 10 minutes, then remove the tonka bean.
Melt the finely chopped chocolate in a bain-marie and, once melted, stir it into the cream. Do not skip this passage, to have a smooth chocolate panna cotta first melt the chocolate then add it to the cream.
Wring gently the gelatine to remove excess water and dissolve it into the chocolate and cream.
Pour the chocolate and cream into single serving ramekins or silicon moulds - be sure to rinse them Leave the panna cotta in the fridge for a few hours or, better, until the next day.
About an hour before serving the panna cotta, chop the strawberries, sprinkle with sugar and add a few chopped fresh mint leaves. Set aside so that they will produce a thick syrup.
To unmould the panna cotta pass a knife on the edges and gently tip the bottom of your mould or immerse the moulds for a few seconds into hot water before unmoulding.
Serve the panna cotta with a few tablespoons of strawberries and drizzle with the strawberry syrup.