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.