In a medium bowl, combine flour, salt, baking soda, and baking powder. Set aside
In another bowl, combine walnuts, both sugars, salt, cinnamon, and melted butter and stir until combined and the walnuts are coated. Set aside
In a large bowl cream the softened butter and granulated sugar together until light and fluffy (3 minutes)
Mix in eggs and vanilla until combined
Mix in yogurt until combined
Gently fold the flour mixture into the wet mixture using a spatula until just combined, making sure to not over mix (the batter will be quite thick). Fold the chopped apples into the batter until just combined
Spread half of the batter into a greased and parchment-lined 9x13 inch baking pan
Sprinkle half of the crumble mixture evenly overtop
Spread the remaining batter over top and top with the remaining crumble
Bake in a 350-degree oven for about 40 minutes, or until a toothpick inserted in the center comes out clean. Allow cooling for 30 minutes prior to slicing and serving.