1. Preheat your oven to 375°F.

2. In a large bowl, mix together the flour and salt for the pie crust. Cut in the cold, diced butter until the mixture resembles coarse crumbs. Gradually add the ice water, mixing just until the dough comes together. Divide the dough into two discs, wrap them in plastic, and chill for 30 minutes before rolling out.

3. In a large skillet, melt the 2 tablespoon of butter over medium heat. Add the sliced Granny Smith apples, sugar, cinnamon, nutmeg, allspice, salt, and lemon juice. Cook the mixture for 5-7 minutes, stirring occasionally, until the apples are slightly softened. Remove the skillet from the heat and set aside.

4. In a small saucepan, melt the 1/4 cup of butter over medium heat. Stir in the brown sugar and heavy cream, and bring the mixture to a simmer. Cook for 2-3 minutes, stirring constantly, until the sauce has thickened. Remove the saucepan from the heat and set the toffee sauce aside.

5. Roll out one disc of the chilled pie dough and fit it into a 9-inch pie pan. Pour the apple filling into the crust. Drizzle the toffee sauce evenly over the apples. Roll out the second disc of dough and place it over the top of the pie. Trim any excess dough and crimp the edges to seal. In a small bowl, whisk together the egg and 1 tablespoon of water, then brush the egg wash over the top of the pie.

6. Bake the Toffee Apple Pie for 45-50 minutes, or until the crust is golden brown and the filling is bubbly. Allow the pie to cool completely on a wire rack before serving.

7. While the Toffee Apple Pie is baking, you can make the creamy vanilla custard sauce. In a saucepan, heat the 2 cup of milk just until it begins to simmer. In a separate bowl, whisk together the egg yolks, sugar, cornstarch, and a pinch of salt. Slowly pour the hot milk into the yolk mixture, whisking constantly. Return the mixture to the saucepan and cook over medium heat, stirring frequently, until the custard has thickened. Remove the saucepan from the heat and stir in the vanilla extract and 1 tablespoon of butter.