Preheat the oven to 350°F. Grease a 9" pie dish and set aside.
Using a hand mixer or a stand mixer fitted with the paddle attachment, cream butter with the sugar until smooth. Add in the flour and salt and beat to combine. Press dough into the prepared pie dish. Bake for 14-15 minutes. Set aside to cool completely.
½ cup (113g) unsalted butter, ½ cup (100g) granulated sugar, 1 ½ cups (222g) gluten free flour blend, ½ teaspoon salt
Butterscotch filling:
In a medium saucepan over medium-high heat, combine the milk, brown sugar, arrowroot, cinnamon, vanilla, and salt. Cook, stirring consistently, until mixture thickens, about 10-12 minutes.
3 cups (720ml) whole milk, 1 ½ cups (300g) brown sugar, ¼ cup (30g) arrowroot, 2 teaspoons cinnamon, 1 ½ teaspoons vanilla extract, ¼ teaspoon salt
Remove from heat and add the butter. Mix until melted. Set aside to cool completely before pouring into cooled crust. Chill in refrigerator for 6 hours or overnight.
5 tablespoons (70g) unsalted butter
Notes
Cornstarch can be used in place of the arrowroot as a 1 to 1 substitute. No need to adjust the amount used.