To make the crust, in a medium bowl or the bowl of a stand mixer, mix together the flour and salt.
Grate the cold butter into the flour mixture, and cut in using a pastry blender or the paddle attachment, until the pieces of butter are no larger than pea-sized.
Add the buttermilk, and mix until the dough just starts to come together.
Turn out onto a floured surface and gently work into a ball, handling as little as possible. Shape the dough into a disc, wrap it in plastic wrap, and refrigerate for at least 20 minutes, or until ready to use.
While the crust is chilling, line a baking sheet with paper towels and lay out the tomato slices in an even layer.
Sprinkle the tomatoes with the kosher salt, then add a layer of paper towels on top. Let rest for 10 minutes, then gently pat the tomatoes to remove the excess moisture. In a small bowl, mix together the mozzarella, mayonnaise, garlic powder, and basil.
Roll out the pie dough on a lightly floured surface to fit your pie pan. Press it gently into the pan and crimp or roll the edges to make a crust.
Place ⅓ of the tomato slices in a single layer on the bottom of the pie dish (a little overlapping is fine). Spread half of the mozzarella mixture over the tomatoes, then layer another ⅓ of the tomato slices over the top. Spread the remaining mozzarella mixture over the top, then top with the last ⅓ of the tomato slices.
Bake at 350°F for 35-45 minutes, until the crust is golden brown.