Preheat the oven to 400 degrees F (200 degrees C). Lightly grease a 10-inch cast iron skillet; line the bottom with a round of parchment paper.

Whisk flour, sugar, baking powder, and salt for shortcake together in a large mixing bowl. Whisk buttermilk, egg yolk, and vanilla together in a glass measuring cup until evenly combined; set aside.

Scatter the butter pieces over the flour mixture. Use a rubbing motion with your fingertips and cut the butter into the flour mixture to distribute. Create a well in the center of the flour mixture. Pour the buttermilk mixture into the center of the well. Use a rubber spatula to mix the flour into the buttermilk mixture to form a moist, shaggy dough.

Transfer the dough to the prepared skillet. Smooth the dough to distribute evenly in the pan. Press the sliced strawberries gently into the surface.

Bake in the preheated oven until golden brown and baked through, 25 to 30 minutes. Cool for 10 minutes.

To prepare the topping, stir together the quartered strawberries, sugar, and lemon zest in a medium mixing bowl to macerate.

To prepare the whipped cream, combine whipping cream, sugar, and vanilla in a chilled mixing bowl. Using an electric mixer fitted with a whisk attachment, beat at medium speed until soft peaks form.

Spoon the strawberries and juices over the shortcake. Serve with whipped cream.