Lightly grease a deep 9-inch cake pan with cooking spray.
Combine chopped durian flesh and coconut milk in a bowl.
Mix flour, water, eggs, sugar, and salt together in another bowl; mix until well-blended. Add durian mixture and mix until combined. Pour batter into prepared cake pan.
Set a round rack in a steamer or large wok or deep skillet over high heat. Pour in water to about 1/2 inch below the rack; bring to a boil.
Place cake pan on rack and cover or tent with aluminum foil, sealing edges tightly. Steam, replenishing boiling water as needed, until a toothpick inserted into the center of the cake comes out clean, about 40 minutes. Carefully remove and let cool before cutting.