Preheat oven to 350°F/160°C. Grease & line 10-inch pan. Sift flour, baking powder; set aside. Cream butter & sugar until fluffy (5-6 mins).
Separate egg whites & yolks. Add yolks gradually. Incorporate yogurt. Gently fold in flour mixture. Add vanilla, cinnamon, nutmeg.
Whip egg whites to soft peaks. Carefully fold into batter. Add diced apples; mix thoroughly. Pour into greased pan; tap to remove air bubbles.
For the topping, blend: Ground cinnamon Melted butter Pecans Caster sugar
Evenly spread topping on batter. Bake 50-55 mins. Insert skewer; it should come out clean. Let cake rest in pan for 20 mins.