For the butterscotch sauce: melt the butter in a sauce pot over medium-high heat. Add the brown sugar, corn syrup, and salt. Bring to a simmer and stir until golden amber in color, about 10 to 15 minutes. Remove from heat and slowly stir in the cream. Remove from the heat and set aside to cool.
For the filling: add the mascarpone cheese and the heavy cream to a large bowl and whish until combined. Add the sugar, vanilla, and salt. Whisk until soft peaks form, then fold in the chocolate chips. Pour the filling into the crust, smoothing out the pie surface. Refrigerate for at least 3 hours and serve drizzled with butterscotch sauce.