Prepare the apples. Peel, core and chop them and put them in a bowl. Toss with the sugar, cinnamon and lemon juice, and set aside.
Generously grease the baking pan with oil and preheat the oven to 350ºF.
Prepare the batter. Mix together the dry ingredients in a large bowl. If using butter, melt it and let cool a bit—you want just barely melted, not sizzling hot. Crack the eggs into another bowl and whisk them, then add the vanilla and buttermilk, oil and butter, and mix together.
Make a well in the center of the dry ingredients. Add the wet ingredients all at once. Fold and mix everything together until everything is just combined. Add the apples and lightly mix. Pour everything into the baking pan and put it in the center of the oven.
Bake about 45 minutes for a metal pan, 55-60 minutes for a glass pan. It is done when the top is lightly golden, the cake just barely begins to pull away from the sides and a tester inserted in the center comes out clean.
Serve warm or at room temperature. If you are feeling lavish, add whipped cream or a scoop of vanilla ice cream. But to me, this is fine on its own.