In a small saucepan bring 1/4 cup whipping cream to a boiling. Remove from heat, add chocolate and butter. Let stand 5 minutes. Stir until smooth; set aside.
In another small saucepan, bring the milk to boiling. While the milk is heating, whisk together the egg yolks, 3 Tbsp. of sugar, the cornstarch and salt. While whisking, very slowly add the hot milk to the bowl. When about one quarter of the milk has been added, add the remaining milk in a steadier stream while continuing to whisk.
Pour the mixture back into the saucepan over low heat. Continue to stir until the custard thickens; 5 minutes.
Add the custard to the melted chocolate and stir gently until blended. Scrape mixture into a bowl. Place bowl into a larger bowl filled with ice and cold water. Keep the custard in the ice bath, stirring frequently, until it is cold to the touch. Remove bowl from the bath.
In a large chilled mixing bowl beat the remaining 1/2 cup cold whipping cream with an electric mixer on medium speed until it holds soft peaks. Sprinkle in the remaining 1 Tbsp. sugar and continue to whip until the peaks are almost firm.
Stir a spoonful of whipped cream into the custard to lighten it. Gently fold in the remaining cream.
Line 1 3/4-inch muffin cups with paper baking cups. Spoon mousse into prepared cups. Freeze for 20 to 30 minutes, then add popsicle sticks or lollipop sticks that have been cut in half. Freeze for 2 hours or until firm.