Make the pie crust:
In a large bowl, combine flour and salt. Add chilled butter and cut it into the flour using a pastry cutter or your hands until it resembles coarse crumbs.

Gradually add ice water, 1 tablespoon at a time, stirring with a fork until the dough begins to come together. Form the dough into a disc, wrap it in plastic wrap, and refrigerate for at least 30 minutes.

Preheat the oven to 375°F (190°C). Roll out the chilled dough on a floured surface and fit it into a 9-inch pie dish. Trim excess dough and crimp the edges. Prick the bottom with a fork. Bake for 15-18 minutes until golden brown. Let it cool completely.

Prepare the butterscotch filling:
In a medium saucepan, melt butter over medium heat. Add brown sugar and salt, stirring until smooth, about 2 minutes. Gradually whisk in milk, bring to a simmer, and cook for 5 minutes, stirring occasionally.

In a small bowl, whisk egg yolks and cornstarch. Slowly add 1/2 cup of the hot milk mixture to the eggs, whisking constantly to temper. Gradually whisk the egg mixture back into the saucepan and cook over medium heat for 5-7 minutes, or until thickened to a custard-like consistency.

Remove from heat and stir in vanilla extract and 1 tablespoon of butter until smooth.

Assemble the pie:
Pour the butterscotch filling into the cooled pie crust and smooth the top with a spatula. Let the pie cool to room temperature, then refrigerate for at least 4 hours, or until fully set.

Serve:
Before serving, top with whipped cream and a sprinkle of toffee bits for added flavor and texture.