Preheat the oven to 400° and brush bunt cake pan with butter.
In a medium saucepan, melt 4 tablespoons of vegan butter. Whisk in the flour and cook over moderate heat for 1 minute. Whisk in the soy milk and cook over moderately low heat until smooth and very thick, about 2 minutes. Stir in the salt and cayenne. Turn off the heat, whisk in the egg yolks, cool slightly. Transfer to a large bowl and stir in the blue cheese, spinach and mushrooms.
In another large bowl, using an electric mixer, beat the egg whites with the cornstarch at medium-high speed until frothy. Increase the speed to high and beat until firm peaks form. With a rubber spatula, gently fold these whipped egg whites into the remaining mixture.
Spoon the soufflé mixture into pan. Bake until the soufflé is puffy and golden brown, about 20-25 minutes. Serve immediately, as indeed even the best soufflés do flatten as they cool!