Preheat the oven to 350 degrees.
Pulse 1 cup of graham crackers with 1 cup of walnuts in a food processor with 2 tbsp of softened vegan butter until the nuts are broken down but not powdery.
Press the crust into an even layer in the bottom of a 10-inch springform pan, place the pan on a baking sheet, and bake the crust for 10 minutes. Remove and set aside while you make the filling.
Make the mango cheesecake filling:
Blend 1/2 cup of cashews with 1/2 cup of any nondairy milk until very creamy.
In a bowl or in the bowl of a stand mixer, beat 20 oz of room temperature vegan cream cheese with 1 cup sugar until well-mixed. Add the cashew cream, 1 tsp green cardamom powder, 3 tbsp of cornstarch and the mango puree. Mix until you have a very smooth, creamy texture.
Before you start baking the cake, place a small, ovenproof pan filled with water on a lower shelf of the oven. Pour the mango filling into the springform pan which should still be standing on a baking sheet (to make it easier to get the pan in and out of the oven).
Place the pan and baking sheet into the oven and bake 30 minutes, then lower the heat to 325 degrees and continue baking another 30 minutes.
When the cheesecake is done, the sides will look firm and the center might still look slightly jiggly, which is okay.
Make the sour cream topping:
About 15 minutes before the cake comes out of the oven, blend 1 cup of vegan sour cream with 1/4 cup sugar and 1/2 tsp of cardamom.
As soon as the cake comes out of the pan, pour the filling on the cake and, using oven mitts, gently tilt the pan around so the topping is evenly distributed over the cake. Return the cake to the oven and continue baking another 15 minutes.
Remove the cake from the oven and let it cook thoroughly on a rack. Once cool, run a knife gently along the edges to loosen the cake from the pan. Then, with the cheesecake still inside the pan, refrigerate it for several hours or overnight until thoroughly chilled.
Unmold, slice and eat.