Preheat oven to 350°F (175°C) and line a 9×5-inch loaf pan with parchment.
In a bowl, whisk flour, sugar, baking powder, and cinnamon until smooth.
In another bowl, whisk eggs, oil, and milk until well combined.
Fold wet ingredients into dry until just mixed; avoid overmixing.
Stir in diced apples and any optional add-ins.
Pour batter into the pan, smooth top, and sprinkle cinnamon-sugar mix.
Bake 50–60 minutes until golden; a toothpick should come out clean.
Cool 10–15 minutes, then lift from pan and cool completely.
Slice and serve warm or at room temperature.
