For the chocolate ganache: Bring the heavy cream to a simmer in a small saucepan over medium-high heat. Remove from the heat and stir in the bittersweet chocolate chips until they melt and the mixture thickens. Line up 4 dessert ramekins and divide the ganache evenly among them. Set aside.
For the butterscotch pudding: Whisk together the brown sugar, cornstarch and salt in a large saucepan over medium-high heat. Slowly whisk in the cream, milk and egg yolks, bring to a boil, and stir constantly until the mixture thickens, about 2 minutes. Remove from the heat and stir in the butter and vanilla extract. Pour on top of the chocolate ganache in the dessert ramekins. Refrigerate for at least 1 hour. Serve chilled.