Place all of the ingredients for the pecan streusel topping in a medium bowl. Using a pastry cutter or your fingers mix the butter into the dry ingredients until you have a mixture resembling coarse crumbs. Set aside.
In a small bowl mix together the diced apple, cinnamon, nutmeg and lemon juice until the apples are well coated. Set aside.
In a medium bowl mix together the sour cream and baking soda set aside.
In the bowl of a stand mixer cream together 1/2 cup sugar and the butter.
Beat in egg yolks two at a time, mixing completely after each addition.
Stir milk and caramel into the sour cream mixture.
Set your mixer on low and alternate adding flour and caramel mixtures. Mix just until combined; do not over mix.
In another bowl beat the egg whites until soft peaks form. Continue beating while gradually adding the remaining 1/4 cup sugar. Beat until stiff and glossy.
Fold 1/3 of the egg whites into the batter to lighten it. Fold in the remainder of the egg whites until just barely combined (some white may show in the batter).
Gently fold in the apple and spice mixture.
Spoon the batter into the prepared muffin tins filling about 2/3 full.
Sprinkle about 1 tbsp of streusel topping over the batter in each cup.
Place the pans in the heated oven for 16-18 minutes or until the tops are golden brown and a toothpick inserted in the center of a muffin comes out with a few moist crumbs.