Preheat oven to 350℉. Grease your baking dish with 2 tbsp of butter.
Combine nuts, brown & white sugars, melted butter, cinnamon, and salt in a bowl until walnuts and sugars are thoroughly coated with butter. Set the crumble aside.
In a medium bowl, whisk together dry ingredients. Set aside.
In a large bowl, use an electric mixer to beat together the white sugar and butter. Once smooth, whisk in 1 egg until smooth, about 2 minutes. Then, mix in the second egg.
Next, whisk in the yogurt and vanilla. Then, add the flour mixture in, and gently combine with a rubber spatula. (Do not over mix).
Fold diced apples into the mixture until just combined.
Spread ½ the batter into the prepared baking dish. Then, evenly scatter ½ crumble mixture over top. Add the remaining batter on top in dollops, then carefully spread. Lastly, sprinkle the remaining crumble on top. Gently press down with a rubber spatula.
Bake for 40 minutes, until a toothpick is inserted and comes clean. Then, allow to cool for 15 minutes.
While cake is cooling, mix icing ingredients together. Drizzle on top and enjoy.