Peel and slice the Granny Smith apples. In a wide pot over medium-high heat, melt 2 tablespoons butter. Add the sliced apples, granulated sugar, cinnamon, nutmeg, allspice, salt, and lemon juice. Stir everything together and cook, stirring occasionally, until the apples are softened, have started to release their juices, but still hold their shape. Avoid overcooking the apples to prevent them from becoming mushy. Transfer the cooked apples and juices to a wide dish or baking pan and spread them out to cool quickly, then place in the refrigerator after cooling slightly.

In a food processor, pulse together all-purpose flour and salt. Add chilled, cubed butter and pulse several times until the mixture resembles small peas. While pulsing, gradually add the ice-cold water until the dough starts coming together in small clumps—add more water if needed. Transfer the dough to a counter, press it into a large ball, then divide into two portions, one slightly larger. Shape each into a flat disc, wrap in plastic, and refrigerate for at least 1 hour until firm.

Melt butter in a small saucepan over medium heat. Add the brown sugar and whisk together. Pour in the heavy cream and whisk until the mixture is emulsified and smooth. Bring it to a boil and cook for 1 to 2 minutes, then remove from heat. Transfer to a bowl and let cool—I like to let the sauce cool slightly so it's easier to drizzle over the apple filling later.

Preheat your oven to 350°F (175°C) and lightly grease a pie plate. Roll out the larger disc of dough on a floured work surface to a circle about 2 inches wider than your pie plate. Lay the dough into the pan, letting excess hang over the edge. In a small bowl, whisk together the egg and water to make an egg wash and set aside. Stir the cornstarch into the cooled apple mixture from Step 1. Fill the pie crust with half the apple mixture, drizzle half the toffee sauce from Step 3, then repeat with the remaining apples and toffee. Roll out the smaller dough disc and cut into strips for a lattice if desired. Brush the edge of the bottom crust with egg wash, then lay half the strips over the pie and weave the other half to form a lattice. Trim excess dough and crimp the edges to seal. Finally, brush the lattice and edges with the remaining egg wash.

Place the filled pie on a baking sheet lined with aluminum foil to catch any drips. Bake for 50 to 60 minutes, until the filling is bubbling and the crust is a deep golden brown. Remove the pie from the oven and let it cool and set before slicing. I always let the pie sit for at least an hour so the filling isn't too runny when cutting.

Set a bowl with a fine mesh strainer aside. In a small bowl, whisk together the egg yolks, granulated sugar, cornstarch, and a pinch of salt until smooth and light in color. Heat the milk in a small saucepan over medium heat just until it’s about to boil. Gradually add the hot milk to the egg yolk mixture, whisking constantly to prevent curdling, then return the mixture to the saucepan. Cook over medium heat, stirring constantly, until the custard thickens and just comes to a boil. Remove from heat, stir in butter and vanilla extract, then strain through the mesh strainer into the prepared bowl. Cover with plastic wrap directly on the surface and cool completely in the refrigerator until ready to serve. For a more fragrant custard, I like to use real vanilla extract here.

To serve, slice the cooled toffee apple pie and place on plates, topping each slice generously with the chilled vanilla custard sauce from Step 6. Enjoy this comforting dessert—trust me, the combination of rich custard and gooey toffee apples is irresistible!