In a large heavy saucepan, stir the sugar and cornstarch together. Stir in the cream and milk, cook over medium heat, stirring constantly for about 10 minutes, or until the mixture thickens and begins to bubble.
Transfer the milk mixture to a glass bowl and add the mint. Set the bowl over a bowl of ice water to cool the mixture down quickly and stir occasionally until cool; discard the mint after 5 minutes.
If you leave the mint in too long the flavor will be too strong.
Cover the mint cream and refrigerate about 4 hours, or until completely cold.
While the milk mixture is cooling, melt the chocolate. This can be done in either the microwave or a double boiler. Cool until the chocolate is barely warm and still fluid.
Freeze the mint cream in an ice cream maker according to directions until frozen.
As the mixture begins to thicken in the freezer quickly drizzle the melted chocolate into the gelato while the ice cream maker is processing. The chocolate will flake apart as it hits the cold gelato, forming bits of chocolate throughout.
Serve immediately, or transfer to a container and freeze until firm, 3-4 hours.