Slice strawberries then add about 1/3 of them to a bowl and mash with a potato masher. Add remaining sliced strawberries and 6 Tbsp sugar and toss. Chill at least 30 minutes.
Preheat oven to 425 degrees. Line a 18 by 13-inch baking sheet with parchment paper.
In a mixing bowl whisk together flour, 1/3 cup sugar, baking powder, baking soda, and salt. Add butter and cut into mixture until it resembles coarse crumbs.
Make a well in center of mixture. In a mixing bowl (or large liquid measuring cup) whisk together buttermilk, 1 whole egg and vanilla extract.
Pour buttermilk mixture into well in center of flour mixture. Fold with a rubber spatula until it comes together in large pieces.
Drop mixture onto a lightly floured surface and turn and press gently about 4 or 5 times until it comes together. Pat into a square about 8 by 8-inches.
Cut into rounds using a 2 1/2-inch round, floured cutter then transfer to lined baking sheet.
Brush tops with egg white then sprinkle evenly with remaining 1 Tbsp sugar. Bake in preheated oven until cooked through and lightly golden brown, about 10 - 12 minutes. Let cool on wire rack 10 minutes.
Pour heavy cream and 1 1/2 Tbsp sugar into a chilled mixing bowl. Using an electric hand mixer whip until stiff peaks form.
To assemble shortcakes, split each biscuit in half using a knife, dollop a spoonful of whipped cream over bottom halves, spoon plenty of strawberries over cream layer then top with second half of biscuit. Serve right after assembling.