Stir chocolate, milk, and salt in heavy medium saucepan over medium heat until chocolate melts. Stir constantly until all sugar in chocolate is dissolved, about 13 minutes.
Add rum to chocolate; using long match, immediately ignite rum. Carefully stir chocolate mixture with long spoon until flames subside.
Transfer chocolate to large bowl. Cool to room temperature.
Using electric mixer, beat 3 cups cream in large bowl until peaks form. Working in 2 batches, fold whipped cream into chocolate mixture.
Divide chocolate mousse among 12 glasses, about 3/4 cup for each. Chill mousse until set, at least 3 hours. (Can be made 1 day ahead. Cover and chill.)
Beat remaining 1 cup cream until peaks form.
Place dollop of whipped cream atop mousse in each glass.
Serve mousse with Cinnamon-Almond Cookies.
*Mexican chocolate is available at some supermarkets and at Latin markets.
