Preheat the oven to 350°F (175°C) and grease two loaf pans.
Whisk the sugar, flour, baking powder, and salt in a medium bowl.
Beat the eggs, milk, and oil in a large bowl until just combined.
Gradually add the dry mixture to the wet mixture until just moist, then fold in the diced apples and cinnamon.
Divide the batter between the loaf pans, filling each about two-thirds full.
Stir the topping sugar and cinnamon together, spoon it over each loaf, and gently swirl it with a knife.
Bake for about 40 minutes, until a toothpick inserted in the center comes out clean.