Combine the cream and corn syrup in a small-medium saucepan and bring to a simmer.
Carefully, pour the hot liquid over the chocolate and allow it to sit for 3 to 4 minutes, or until the chocolate has melted.
Whisk, add kahlua (if using) combine well and allow sauce to slightly cool.
Mousse:
Chill the mixer bowl and whisk attachment.
Place heavy cream and sugar in the chilled bowl; using an electric hand mixer, whisk on high-speed until stiff peaks form. Refrigerate until ready to use.
Melt white chocolate in a medium heatproof bowl in the microwave on low power setting in 30-second intervals, stirring in between, until the chocolate is smooth and completely melted.
Using a rubber spatula, fold in 1/4 of the whipped cream into the melted white chocolate until smooth. Add the remaining whipped cream to the chocolate mixture and gently fold until combined.
Divide chocolate sauce among 6 serving cups, then pipe or spoon mousse over layer of sauce. Place in refrigerator until chilled. Dust with cocoa powder and grated chocolate.
