Heat oven to 350 F. Lightly coat 2 muffin tins with cooking spray. In a mixing bowl, combine the yogurt, eggs, oil and vanilla. In a medium bowl, combine the flour, 1 cup sugar, oats, flaxseed, 2 teaspoons cinnamon, baking powder and salt. Turn the mixer to low speed. Slowly add the dry ingredients to the wet ingredients. Mix until just combined. Batter should be lumpy. Fold in the apples with a spatula.

Scoop 1/4 cup of batter into each muffin well. In a small bowl, combine remaining 2 tablespoons sugar and 1/4 teaspoon cinnamon and sprinkle over the batter in each muffin well. Bake for about 22 minutes or until tops are golden brown and toothpick comes out clean when inserted.