Preheat the oven to 350°F/180°C, line a 8 or 9 inch cake pan with parchment paper and cake round and spray with non stick spray.
In a bowl, combine the dry ingredients (flour, baking powder, baking soda, cinnamon, and salt). Set aside.
In a separate mixing bowl, beat butter with sugar until smooth and creamy, then mix in eggs and yogurt and beat again.
Add the dry ingredients to the wet ingredients, and mix until just combined. Do not over mix.
Gently fold in the apples. Then pour the batter into the prepared cake pan.
To make the streusel, in a small bowl combine the streusel ingredients (sugar, cinnamon, and walnuts), and sprinkle mixture over cake batter.
Bake in the preheated oven for 50 minutes or until a toothpick comes out clean from the center of the cake.
Remove from the oven, and allow to cool in the pan for 10 minutes. Then remove the cake from the cake pan, and cool on a wire rack completely.
When cooled, serve as is or sprinkle with powdered sugar, slice and serve.