Preheat the oven to 375°F (190°C). Line a large baking sheet with parchment paper so the fritters do not stick and bake evenly.
In a large bowl, whisk together the flour, granulated sugar, baking powder, cinnamon, nutmeg, and salt until the mixture looks uniform and the spices are evenly distributed.
In a medium bowl, whisk the milk, eggs, vanilla extract, and melted butter until smooth. Make sure the butter is not hot so it blends evenly with the eggs.
Pour the wet ingredients into the dry ingredients. Stir gently with a spatula just until the flour disappears. The batter should be thick and slightly lumpy. Overmixing will make the fritters dense.
In a small bowl, toss the diced apples with brown sugar and cinnamon until fully coated. This helps soften the apples and adds flavor throughout the fritters.
Fold the apple mixture into the batter using slow, gentle turns. Stop as soon as the apples are evenly mixed to keep the batter light.
Scoop the batter into 10 to 12 mounds on the prepared baking sheet. Leave space between each fritter so heat can circulate and they bake evenly.
Bake for 22 to 25 minutes, or until the tops are lightly golden and the centers feel set when gently pressed. If needed, bake an additional 2 minutes.
Remove the fritters from the oven and let them rest on the baking sheet for 5 minutes. This allows them to finish setting and keeps the texture soft.
If using the glaze, whisk powdered sugar, milk, and vanilla until smooth. Drizzle over warm fritters for a thinner glaze or cooled fritters for a thicker finish.