Preheat your oven to 180°C (350°F) and line a muffin tin with paper liners or grease it well.
Peel and dice the apples into small, uniform pieces. Set aside a few slices for topping if you like.
In a large mixing bowl, toast the oats lightly in a dry skillet until fragrant, then transfer to the bowl.
Add the brown sugar, cinnamon, baking powder, and salt to the oats, whisking to combine evenly.
In a separate bowl, whisk the eggs, then stir in the melted butter and vanilla extract until smooth.
Pour the wet mixture into the dry ingredients and gently fold until just combined. The batter will be slightly lumpy.
Fold in the diced apples evenly, distributing them throughout the batter.
Spoon the batter into the prepared muffin cups, filling each about three-quarters full. Top with a few apple slices if desired.
Bake for 20-25 minutes, until the muffins are golden brown and a toothpick inserted in the center comes out clean. The house will fill with the inviting aroma of cinnamon and baked apples.
Remove the muffins from the oven and let them cool in the tin for 5 minutes, then transfer to a wire rack to cool completely.
Enjoy the muffins warm or at room temperature, perhaps with a drizzle of honey or a pat of butter for extra coziness.
