Place the milk in a heavy, small saucepan. Sprinkle the gelatin over and let stand for 5 minutes to soften the gelatin. Stir over medium heat just until the gelatin dissolves, but the milk does not boil, about 2 minutes. Add the cream, espresso powder, sugar, and salt. Stir over low heat, until the sugar dissolves, about 3 more minutes. Remove from the heat and let cool slightly. Pour the cream mixture into 2 martini glasses, dividing equally. Cover and refrigerate, stirring every 20 minutes during the first hour. Chill until set, at least 6 hours and up to 2 days.
When ready to serve, use a vegetable peeler on the chocolate blocks to create about 1 tablespoon each of the white and dark chocolate shavings. Sprinkle the shavings over each panna cotta and serve.