In a medium saucepan, whisk together 2/3 cup sugar, the cornstarch, cocoa powder and salt. In a small bowl, whisk together half-and- half and yolks. Add to sugar mixture in saucepan and whisk vigorously over medium heat until smooth. Add chocolate and bring to a boil; cook, stirring constantly, until chocolate melts and mixture thickens. Remove from heat and stir in butter. Let cool slightly, then pour into a shallow 1-1/2- to 2-qt dish. Cover with plastic wrap and refrigerate until chilled, at least 2 hrs or overnight.

Meanwhile, for topping, line a baking sheet with parchment paper. In a small saucepan, combine remaining 2/3 cup sugar and 1/4 cup water. Stir over medium-high heat until sugar is dissolved. Brush down sides of pan with water to dissolve any sugar crystals. Stop stirring and let mixture caramelize to medium-amber color, about 4 min. Quickly pour syrup onto prepared baking sheet and, using the back of a spoon, spread into a thin layer. Let cool completely, then crack into shards.

To serve: Arrange shards over creme and serve immediately. Makes 4-1/2 cups.