In a large bowl, whisk together the flour, baking powder, baking soda, sugar and salt. Using a cheese grater or pastry cutter, cut the cold butter into the flour mixture until the butter is the size of small peas. Stir.
Add the beaten egg, vanilla extract and buttermilk to the flour mixture. Stir until combined.The dough will be sticky. Cover the bowl with plastic wrap and chill in the refrigerator for 30 minutes. Start on the filling.
Preheat oven to 400 degrees.
In a large 12 inch cast iron skillet (or oven safe skillet), melt the 2 tablespoons of butter over medium high heat. Add the sliced strawberries and sugar. Bring the mixture to a gentle boil and cook stirring often for 5 minutes until the strawberry juices release and the mixture is bubbly. Turn the heat down to low and add the cornstarch, lemon juice, vanilla extract and strawberry jam, if using. Set aside on low while you finish the shortcakes.
Remove the shortcake dough from the refrigerator. On a lightly floured work surface empty the dough out. With floured hands, knead the dough a few times by folding it over on itself and repeating. The dough will be sticky, but if it's too sticky to work with, add ⅛ cups of flour at time until it's easier to work with. Roll the dough out into a 7 inch disc about ½ inch thick. Using a biscuit cutter or drinking glass, cut out the biscuits. You should have around 10-12 biscuits.
Arrange the biscuits on top of the strawberry filling. Beat the remaining egg in a small bowl with 1 tablespoon of water and brush the tops of the shortcakes with the egg wash. Sprinkle with sparkling sugar or regular sugar if desired. Bake for 30-35 minutes, or until the tops of the biscuits are golden. Allow to cool for 10 minutes and serve with vanilla bean ice cream or a dollop of fresh whip cream. ENJOY!