Preheat the oven to 400° F
Butter and flour (or line bottom with parchment paper) one 9" square baking dish (or tart pan)
Chop the apples and place them in a bowl of water, along with the juice of one lemon. Set aside.
In a large bowl, combine the flour, baking powder, cinnamon, sugar and salt in a large bowl.
In a separate and smaller bowl, whisk the eggs and the buttermilk.
Whisk in the melted butter.
Pour the buttermilk mixture over the flour mixture and stir until barely combined (be careful not to over-mix)
Drain the apples, shake off any excess water, and fold them into the cake batter.
Spoon the batter into the prepared pan, pushing it out toward the edges. Sprinkle with the large grain sugar.
Bake for about 20-25 minutes or until cake is just set and a touch golden on top.
(I like this cake every-so-slightly under-baked, just barely, remember it will cook for a little while after you remove it from the oven)