Peel and core the apples. Then cut them into fine cubes. Drizzle the apple pieces with a squeeze of lemon juice and then sprinkle ½ teaspoon of cinnamon on top. Mix everything well and set aside, covered.
Combine the oatmeal with the yogurt and ¼ cup milk.
In a separate bowl, sift the flour and mix it with the baking powder, salt and remaining cinnamon.
Separate the egg whites. Cream together with the melted butter. Then beat in the sugar and vanilla extract.
Fold in the oat mixture.
Alternate stirring in the flour and remaining milk, being careful not to overmix. Stir just enough to combine—do not stir so much that the batter becomes overdone.
Finally, fold in the apples and divide the batter among 12 muffin cups or a greased muffin tin.