Make the Filling:
In a medium saucepan, combine the cream and milk and bring to a boil. Remove from the heat. Add the chocolate and let stand until melted, 5 minutes. Whisk until smooth. In a small bowl, whisk the egg with the egg yolk, then whisk in 1/2 cup of the chocolate cream. Thoroughly whisk this custard into the chocolate cream in the saucepan. Pour the filling into the tart shell and bake until set around the rim but still slightly jiggly in the center, 30 minutes. Let the tart cool on a rack for 10 minutes, then refrigerate until chilled, 2 hours.