In a heavy saucepan, combine the milk, sugar, powdered milk, vanilla and corn syrup, and bring to a boil. Whisk until the sugar and powdered milk have dissolved. Remove from the heat, pour into a bowl, and allow to cool—or chill in the refrigerator—until mixture is room temperature. Add the salt.
Freeze in an ice-cream maker according to manufacturer's instructions.
Shortly before the gelato is ready, melt the chocolate in a microwave-safe bowl at 50-percent power, or set in a heatproof bowl over a saucepan of simmering water, for about 2 minutes. Cool to 100 degrees F (tepid).
When the gelato has reached the desired texture, drizzle in the chocolate in a very fine stream while continuing to run the machine. You can control the stream by using a spatula for the drizzling. If the stream is too thick or too fast, the chocolate will form into clumps instead of flecks.
Transfer the gelato to the freezer container and freeze for at least 2 hours to firm up before serving.