In the top of double boiler set over medium high heat, melt the chocolate. Remove from the heat.
In a medium saucepan, bring the cream to a gentle boil. Remove from the heat.
In a small mixing bowl, whisk together the egg yolks and sugar until slightly thickened. Whisk 1/2 cup of the hot cream into the egg mixture. Whisk in the coffee powder. In a slow, steady stream, gradually add the egg mixture to the pan of hot cream. Cook over moderately low heat, stirring occasionally, until the mixture thickens enough to coat the back of a spoon and reaches 170 degrees F on an instant-read thermometer, 3 to 5 minutes.
Remove the pan from the pan from the heat and strain the mixture into a clean bowl through a fine mesh strainer, pressing against the strainer with the back of a spoon. Add the melted chocolate and vanilla. Pour into a container and cover with waxed paper or plastic wrap, pressing down against the surface to keep a skin from forming. Chill in the refrigerator for at least 2 hours.
When cold, pour into the bowl of an ice cream machine. Churn and freeze according to manufacturer's instructions. When the ice cream has come together, transfer to an airtight container and place in the freezer until ready to serve.
Yield: 3 1/2 cups