Dark Chocolate-Caramel Cake with Gold-Dusted Chestnuts

Combine milk chocolate and bittersweet chocolate in medium bowl. Stir sugar, 2 tablespoons water, and cinnamon stick in heavy medium saucepan over medium-low heat until sugar dissolves. Increase heat and boil without stirring until syrup turns deep amber, occasionally brushing down sides of pan with wet pastry brush and swirling pan, about 6 minutes (time will vary depending on size of pan). Add cream and salt (mixture will bubble vigorously). Bring caramel to boil, whisking until smooth and caramel bits dissolve, about 1 minute. Discard cinnamon stick. Pour hot caramel over chocolate; stir until chocolate is melted and mixture is smooth. Let stand until completely cool, stirring occasionally, about 1 hour. Using electric mixer, beat butter in large bowl until fluffy. Beat in chocolate mixture in 4 additions. Cover and refrigerate ganache overnight.