In a food processor pulse walnuts, oats, coconut sugar, baking powder, cinnamon, cardamom, and salt until well ground. Add vegan butter, and pulse again until dough like in texture.
Preheat oven to 350F/175C. Line a baking dish with parchment paper (I used an 8x11 inch baking dish). Add 1/3 of the dough mixture to the bottom of the dish and push to make an even layer. Reserve other 2/3rds of dough for the crumble topping. Place baking dish in the oven and cook base for 10 minutes, or until lightly golden.
While you're waiting for the base to cook, prepare your apple layer. Thinly slice apples and toss into mixing bowl with maple syrup, flour, cinnamon, nutmeg, cardamom, and vanilla extract. Toss gently to combine.
Spread apple butter in an even layer over pre-cooked base, then arrange spiced apple slices overtop in an even layer. Sprinkle remaining dough overtop of apple slices in clumps. Place back in the oven to bake for another 30-40 minutes, or until golden and bubbling.
Allow crumble to cool entirely (approx. 1 hour) before cutting into bars. (I cut mine into 12 squares).