Place the sliced strawberries in a bowl and sprinkle with the sugar. Stir to combine, cover, and refrigerate for 1 to 2 hours to allow the berries to macerate.
For the Shortcakes:
Place an oven rack in the lower-center position of the oven. Preheat the oven to 425°F. Line a baking sheet with parchment paper and set aside.
Measure the flour, sugar, baking powder, and salt into the bowl of a large food processor. Pulse to combine. Add the cold butter and pulse (1-second pulses) until the butter is incorporated and the mixture resembles coarse sand.
In a measuring cup, use a fork to whisk together the milk and the egg. Slowly pour the milk mixture through the tube of the food processor while pulsing (1-second pulses) until the dough just forms into large clumps. Turn out the dough onto a floured surface and gently knead until it comes together.
Use your hands to form the dough into a rough rectangle measuring approximately 7 ½- by 5-inches (and ¾- to 1-inch thick). Use a floured pastry cutter to cut the dough into six equal squares. Arrange them on the parchment paper-lined baking sheet with 1 to 2 inches between each piece. Brush the tops with the beaten egg white and sprinkle with sparkling or granulated sugar.
Bake for 10 to 12 minutes or until the shortcakes are puffed up and golden brown. If the tops are getting too dark before the centers are done, loosely lay a sheet of foil over the tops for the last few minutes of baking time. Transfer the baking sheet to a cooling rack and allow to cool for 10 to 15 minutes or until they are still warm but cool enough to handle.
For the Whipped Cream:
While the shortcakes are cooling, measure the heavy cream into a bowl. Add the vanilla and powdered sugar. Using an electric mixer, start on low speed then increase to medium-high and whip until stiff peaks just form (do not overbeat or the cream will start to curdle). Set aside.
Slice each shortcake in half like a biscuit. Place the six bottom halves on individual plates (or on a large serving platter) and top with macerated strawberries, evenly dividing the berries between the shortcakes. Add a generous dollop of whipped cream on top of the strawberries on each shortcake. Carefully lay the shortcake tops over the whipped cream, garnishing the tops with remaining whipped cream, if desired. Serve immediately.
Whipped cream gets the best volume when you use very cold cream plus a chilled bowl and beaters. I typically pop my bowl/beater in the freezer for 10 minutes before I make my whipped cream.