In a large bowl, combine the flour and cold butter cubes. Using a pastry cutter or your fingers, work the butter into the flour until it resembles coarse crumbs.
Gradually add ice water and mix until the dough comes together. Divide it into two equal parts, shape into discs, and refrigerate for at least 30 minutes.
Preheat your oven to 425°F (220°C).
In a separate bowl, mix the sliced apples, sugar, cinnamon, salt, lemon juice, and cornstarch.
Roll out one of the dough discs on a floured surface to fit your pie dish. Add the apple filling.
Roll out the second dough disc and place it over the filling. Trim the excess dough and crimp the edges to seal.
Brush the top crust with beaten egg and make a few small slits for steam to escape.
Bake for 45-50 minutes or until the pie is golden brown and the filling is bubbling.
Let it cool slightly before serving. Enjoy Grandma's Apple Pie with a scoop of vanilla ice cream for the full experience!