Peel, core and chop the apples into about 1-inch chunks. Add to a bowl and toss well with the cinnamon and sugar. Set aside.
Preheat the oven to 175 C. Line a 9x5 inch loaf tin with non-stick baking paper, leaving a little overhang for easy removal.
In a mixing bowl, whisk the melted butter and sugars together until well combined, then add the orange juice, vanilla and eggs. Whisk again until smooth and the eggs are incorporated. A balloon whisk is useful here, a hand mixer is not necessary but you can use one on low speed if you prefer.
Sift in the flour, baking powder and salt (if using). Fold it gently into the liquid ingredients using a spatula until incorporated. The batter is on the thicker side but should be smooth.
Spoon half the batter into the loaf tin, smoothen the top and spread half the apples over it in an even layer. Top with the remaining batter, smoothen and finish with the remaining apples.
Bake for 1 hour, then an additional 5 to 10 minutes until a toothpick poked into the center of the cake comes out with no crumbs. Some apple juice might create a gooey appearance, that's ok.
Allow the cake to cool for 30 minutes in the tin, then carefully lift it out and place on a wire rack to cool completely. Slice into 6 thick slices (the apples make it harder to cut thin slices) using a sharp knife and then if you like, slice each one into 2 smaller pieces so they're easier to eat.
Store at room temperature for a day after which it's best stored in the fridge for a week. I recommend reheating individual pieces and drizzling a little cream on top! Happy baking :)