4 tablespoons (56 g) unsalted butter, chopped
1/2 cup (109 g) packed light brown sugar
3/4 cup (150 g) granulated sugar
3/4 cup (6 fluid ounces) heavy whipping cream
1/2 teaspoon pure vanilla extract
1/2 teaspoon kosher salt (or more to taste)
5 1/2 tablespoons (48 g) xanthan gum-free gluten free flour blend (32 grams superfine white rice flour + 10 grams potato starch + 6 grams tapioca starch/flour)
6 tablespoons (72 g) sugar
1/8 teaspoon kosher salt
3 cups (24 fl. oz.) milk (any kind), at room temperature
4 egg yolks (120 g total), at room temperature
2 tablespoons (28 g) unsalted butter, chopped
2 teaspoons pure vanilla extract
First, make the butterscotch sauce.* In a medium-size, heavy-bottom saucepan, melt the butter over medium heat. Add the brown sugar, and stir until combined. Stir in the granulated sugar, and cook over medium-low heat until smooth, stirring frequently. The mixture will clump, and eventually melt after about 3 minutes. Remove the saucepan from the heat and, whisking constantly, add the cream. The mixture will bubble quite a lot, and the sugar may seize. It will melt again, though. Continue whisking until the bubbling subsides. Return the saucepan to medium heat and, whisking occasionally, bring to a simmer. Continue to cook, whisking occasionally, for about 5 minutes or until the mixture is slightly reduced and coats the back of a spoon. Remove from the heat, and add the vanilla and salt. Set aside to cool.
*The butterscotch sauce can be made up to 5 days ahead of time and stored, covered, in the refrigerator. As it will harden when chilled, before using in the pudding and to serve, the sauce must be warmed in a saucepan on the stovetop or in the microwave, just until melted and smooth, but not hot.
Make the pudding. In a medium-size heat-safe bowl, place the flour blend, sugar and salt, and whisk to combine well. Add 1/2 cup (4 fl. oz.) of the milk, and then the egg yolks, whisking to combine after each addition. The mixture should be smooth. Set it aside. In a medium-size heavy-bottom saucepan, place the remaining 2 1/2 cups (20 fl. oz.) milk and bring to a simmer over medium heat. Once the milk reaches a simmer, remove the saucepan from the heat and slowly trickle the hot milk into the bowl with the egg mixture, whisking constantly to combine. Pour the whole mixture back into the saucepan and return to the heat. Cook, whisking constantly, over medium-high heat until the whisk leaves a visible trail in the pudding as you whisk it (2 to 3 minutes). Remove the pan from the heat, add the chopped butter and the vanilla, and about half of the butterscotch sauce, and stir until the pudding is smooth. Pour into a large bowl and place plastic wrap directly on the surface of the pudding to prevent it from forming a skin. Set aside to cool briefly.
Assemble the individual servings. Pour about half of the remaining butterscotch sauce into a layer on the bottom of each of 6 small containers, each about 6 ounces in capacity. Divide the pudding evenly among the containers, and cover with the remaining butterscotch sauce, evenly divided among them. Allow to cool to room temperature, uncovered. The pudding can be served at room temperature or covered and stored in the refrigerator for up to 3 days before serving. If chilled, the butterscotch sauce will harden a bit.