In a medium saucepan, whisk together sugar, cornstarch, cocoa, and salt. In a 4-cup measuring cup, combine 1 1/4 cups heavy cream with milk and vanilla. Whisk 1 cup cream mixture into dry ingredients until cornstarch is completely dissolved. Whisk in remaining cream mixture until smooth.
Place saucepan over medium heat. Cook, whisking constantly, until mixture comes to a boil and thickens, about 4 to 5 minutes. Add chocolate, and cook, whisking until chocolate is melted, about 1 minute. Remove from heat, and whisk in butter until melted. Using a 3-ounce ladle, pour pudding into 6 dessert cups. Refrigerate until completely set, about 1 hour.
Whip remaining cup of heavy cream to soft peaks, and serve with the pudding.
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