Preheat oven to 350°F. Line an 8" cake pan with parchment paper.
Crumb topping: In a bowl, combine flour, brown sugar, cinnamon, and pecans. Add in chunks of softened butter/coconut oil (or pour over melted butter/oil) and mix just until all the flour is hydrated. Avoid over-mixing or the topping will become pasty. Place in the fridge while you prepare the rest of the cake.
Apple filling: Place chopped apples in a large bowl and toss with cinnamon, nutmeg, lemon juice, and caramel. Mix until the apples are evenly coated with caramel.
Cake: In a large mixing bowl, whisk together flours, cornstarch, sugars, salt, cinnamon, nutmeg, baking powder, and baking soda.
In a separate bowl, whisk milk, applesauce, oil, vinegar, and vanilla extract. Pour wet ingredients into dry ingredients and use a spatula to fold the ingredients together to form a thick batter.
Assemble cake: Pour cake batter into your lined cake pan. Spread apple filling on top and sprinkle the crumb topping all over.
Bake: Bake for 60-70 minutes until a toothpick inserted in the center of the cake comes out clean. Check the cake around the 40 minute mark. If the crumb topping is browning a lot, tent the cake with foil to prevent burning.
Cool: Place the cake pan on a wire rack to cool completely. Optionally, you can drizzle with extra caramel sauce before serving.
Notes
Softened vs melted butter for crumb topping: Softened butter will create more chunks. Melted butter will yield finer crumbs. Caramel: I used my homemade vegan caramel. If it's firm from being in the fridge, microwave for about 10 seconds to soften. Whole Wheat Flour: I enjoy the flavor combo of apples and whole wheat flour, but you can replace the whole wheat flour for more all purpose flour.