Stir white chocolate, 1/4 cup cream and 2 tbsp syrup in saucepan over very low heat until chocolate is melted and smooth.
Pour into a bowl and allow cooling to lukewarm.
Beat 3/4 cup cream with electric mixer to firm peaks.
Fold cream into the white chocolate mixture in 2 batches.
Divide mousse among 4 custard cups.
Cover and refrigerate until firm, about 4 hours.
Can be prepared up to 2 days ahead.
Bring remaining 6 tbsp cream and remaining 2 tbsp corn syrup to simmer in heavy saucepan over high heat.
Reduce heat to low, add semi-sweet chocolate and stir until melted and smooth.
Cool to room temperature.
Spoon enough sauce over each mousse to cover completely.
Garnish with mint leaves, and with a dollop of chocolate whipped cream.