Melt the vegan butter in a saucepan over medium heat. Add the shallots and sauté for 4 to 5 minutes, until translucent. Sprinkle in the salt after about 2 minutes
Add the garlic and sauté for 1 minute. Sprinkle in the whole peppercorns and allow them to sit in the hot oil for 3 to 4 minutes before adding the brandy
Let the brandy bubble away for a minute, then add the tamari. Simmer it all for 3 to 4 minutes, until you get a thick liquid in the saucepan
Pour in the dairy-free cream and vegetable stock, then stir in the mushroom powder (if using). Add a lid to the saucepan and bring the sauce to a simmer. Cook for 10 minutes over low heat, stirring frequently, then season to taste with salt and black pepper
Run the sauce through a fine-mesh sieve to remove the whole peppercorns and shallots. You can also leave them in the finished sauce. Make it the way you prefer! Mix the cornstarch with water to create a slurry, add it to the sauce and stir until the sauce thickens