Prep the pan and oven. Heat the oven to 350°F (175°C). Line a 12-cup muffin tin with paper liners or grease well so the cups release easily.

Mix the dry ingredients. In a large bowl, combine rolled oats, baking powder, cinnamon, and salt. Stir to distribute the spices evenly.

Whisk the wet ingredients. In another bowl, whisk milk, eggs, maple syrup, applesauce, and vanilla until smooth.

Combine. Pour the wet mixture into the dry and stir until the oats are well coated. Fold in the diced apple and any optional add-ins.

Rest briefly. Let the mixture sit for 5 minutes. This allows the oats to soak up some liquid, giving you a better texture.

Fill the muffin tin. Divide the batter evenly among the 12 cups, filling nearly to the top. If using, sprinkle a pinch of turbinado sugar over each for a crunchy top.

Bake. Bake for 24–28 minutes, until the edges are golden and the centers are set. A toothpick should come out mostly clean.

Cool. Let the oatmeal cups cool in the pan for 10 minutes, then transfer to a wire rack to finish cooling. They firm up as they cool.

Serve. Enjoy warm, or reheat later. They’re great plain or with a smear of peanut butter, almond butter, or a dollop of yogurt.