Preheat your oven to 350°F (175°C). Prepare a muffin tin by lining it with paper liners or by greasing it with cooking spray to prevent sticking.
In a large mixing bowl, thoroughly combine the rolled oats, baking powder, ground cinnamon, and salt.
In a separate bowl, whisk together the applesauce, honey (or maple syrup), egg, and pure vanilla extract until the mixture is smooth and well blended.
Gradually pour the wet mixture into the bowl with the dry ingredients. Stir gently until everything is just combined. Carefully fold in the diced apples and optional nuts, as well as the raisins or dried cranberries, if using.
Evenly distribute the oat mixture into the prepared muffin cups, filling each one about three-quarters full to allow for rising.
Place the muffin tin in the preheated oven and bake for 25-30 minutes, or until the tops are golden brown and the centers have set.
Let the oatmeal cups cool in the tin for approximately 5 minutes. Afterward, transfer them to a wire rack to cool completely and firm up.
Notes
For an inviting serving, enjoy these oatmeal cups warm, drizzled with a touch of extra honey.