Preheat oven to 350°F (180°C).

1. Prepare the topping: In a medium bowl, combine brown sugar, flour, butter and cinnamon. Use a pastry blender or fork to mix until mixture is coarse and crumbly and the butter is well incorporated. Stir in the nuts (if using) and mix well. Set aside.

2. Prepare the cake: In another bowl, combine flour, baking powder and baking soda.

3. In a large mixer bowl, cream butter and sugar on medium speed until smooth and fluffy. Beat in eggs, one at a time, beating well after each addition. Add vanilla and yogurt and beat on low speed until blended. Beat in flour mixture, beating on low speed until well blended.

4. Spoon half the batter into the prepared baking pan. Sprinkle evenly with ¼ cup (50 mL) of the crumb topping, then the apples, and then ½ cup (125 mL) of the crumb topping. Spoon on the remaining batter and top with the remaining crumb topping.

5. Bake in preheated oven for 55 to 60 minutes or until a toothpick inserted in the center comes out clean and dry. Let cool in pan on a wire rack for 10 minutes. Put a large plate over top and carefully invert. Remove pan. Place another plate on top of cake and invert so that crumb topping is facing up. Let cool completely.