In a medium soup pot, bring vegetable stock with mushrooms to a boil over high heat. Remove from heat and let mushrooms steep for 30 minutes. Transfer to large spouted measuring cup and set aside.
Once cool enough to handle, remove mushrooms from stock and finely chop.
Add butter and olive oil to the empty pot and melt over medium-high heat. Add shallots and mushrooms. Cook, stirring, until well browned, 8 to 10 minutes.
Sprinkle in flour and cook, stirring, until golden brown, 3 to 5 minutes.
Slowly add the white wine and stir until combined with the flour. Sprinkle in nutritional yeast. Slowly whisk in stock, a little at a time. Add soy sauce, marmite, and a few grinds of pepper.
Simmer 2 to 3 minutes, uncovered, until thickened. Taste for seasoning and add salt or additional pepper if needed. Serve as is, or pass it through a fine mesh strainer.