How to Make It
In a food processor, combine the flour, cornstarch, sugar and salt. Add the butter and pulse in 1-second bursts until the mixture resembles coarse meal. Combine the milk and vinegar and drizzle it on top. Pulse in 1-second bursts until the dough just comes together. Turn the dough out onto a work surface, gather up any crumbs and pat into a disk. Wrap in plastic and refrigerate until chilled, about 30 minutes.
On a floured work surface, roll out the dough to an 11-inch round, a scant 1/4 inch thick; ease it into a 9-inch glass or ceramic pie plate. Trim the overhanging dough to 1 inch and fold it under itself. Crimp decoratively and chill the crust until firm, about 15 minutes.
Preheat the oven to 425°. Line the crust with parchment paper and fill with pie weights or dried beans. Bake in the lower third of the oven for about 15 minutes, until the crust is barely set. Remove the parchment and pie weights. Cover the edge of the crust with strips of foil and bake for about 15 minutes longer, until the crust is just set but not browned. Press the bottom of the crust lightly to deflate it as it puffs; let cool. Lower the oven temperature to 350°.
In a medium saucepan, boil the cider until it's reduced to 1/2 cup, about 10 minutes. Transfer to a bowl and let cool. Whisk in 3/4 cup of the sugar, the sour cream and salt, then whisk in the eggs.
Pour the custard into the pie shell without removing the foil strips. Bake the pie in the lower third of the oven for 35 to 40 minutes, until the custard is set around the edge but the center is slightly jiggly. Let the pie cool completely.
In a medium bowl, using an electric mixer, beat the heavy cream with the remaining 1/4 cup of sugar and the cinnamon until firmly whipped. Mound the cream on the pie, cut into wedges and serve.
Baked apple slices.