1. Melt milk chocolate in the top of a double boiler or in a metal bowl set over a pot of barely simmering water, stirring frequently until just melted. Remove bowl from heat and stir chocolate until smooth. Chocolate should be just warmer than body temperature. Cool slightly.
2. Whisk egg yolks in a medium bowl. Bring milk to a simmer in a medium saucepan. In a slow stream, whisk hot milk into egg yolks. Whisk constantly so eggs don't curdle. Return mixture to saucepan. Stir with a wooden spoon over low heat until custard thickens enough to coat the back of the spoon, about 6 to 10 minutes (do not boil). Remove pan from heat. Whisk in melted chocolate and vanilla. Let cool at room temperature for 10 minutes, then refrigerate about 40 minutes, until slightly thickened.
3. Using an electric mixer on medium high, beat cream in a medium bowl until soft peaks form. Gently fold cream into chocolate mixture until thoroughly combined. Divide mousse among 6 dessert bowls or one-cup ramekins. Cover bowls loosely with plastic wrap and chill at least 4 hours. (Can be made 1 day ahead; keep refrigerated.) Serve chilled, garnished with a dollop of whipped cream and a sprinkling of cinnamon, if desired.
Source: fresh magazine January, February 2010