Preheat your oven to 350°F (175°C) and grease the loaf pan with non-stick spray or butter.
In a large mixing bowl, whisk together the all-purpose flour, granulated sugar, baking powder, salt, and ground cinnamon.
In another bowl, whisk together the eggs, milk, vegetable oil, and vanilla extract.
Pour the wet ingredients into the dry ingredients and stir gently until just combined. Be careful not to overmix.
Fold in the diced apples.
In a small bowl, mix the brown sugar with 1 tablespoon of cinnamon.
Pour half of the batter into the prepared loaf pan and spread evenly. Sprinkle half of the brown sugar-cinnamon mixture on top.
Add the remaining batter over the cinnamon layer and then sprinkle the rest of the brown sugar-cinnamon mixture on top. Use a knife to swirl the mixture through the batter gently.
Bake in the preheated oven for 50-55 minutes, or until a toothpick inserted in the center comes out clean.
Allow the bread to cool in the pan for about 10 minutes before transferring it to a wire rack to cool completely.
For the topping, whisk melted butter, powdered sugar, milk (or water), and vanilla extract in a small bowl until smooth. Drizzle over the cooled bread before slicing.