First you want to trim the gills out of the under side of the mushrooms and get them as flat as possible. Then either in a large bowl, or in a large ziploc bag, make the marinade.
Mix together the balsamic vinegar, broth, garlic, salt, rosemary and thyme. Put the mushrooms in the marinade, and let marinate for at least 2 hours, but the longer the better.
When you are ready to make the mushrooms, make the spice rub. In a small mixing bowl, stir together the garlic powder, black pepper, salt and onion powder. Then pour the olive oil into the seasoning blend and mix together to make a paste.
Heat a large cast iron skillet on medium high, add the vegan butter to the skillet and then rub each side of all of the mushrooms with the spice paste.
Place the mushrooms in the pan. Sear on each side for 2-3 minutes. Then place something heavy on top of the mushrooms, like a sheet pan or plate with cans on top to press the mushrooms down into the pan. Let cook for about 5 minutes, reducing heat as needed. Flip and press back down and cook for another 5 minutes or so until the mushrooms are brown, juicy and have a nice crust.
Remove them from the pan, but don't wipe out the pan. Reduce heat to medium low. Add the vegan butter for the sauce into the pan. Let it melt, and then add the coconut cream, bourbon and a pinch of salt and pepper. Whisk to combine, then bring to a simmer, reduce heat to low and simmer for about 10 minutes or until the sauce is thick and the alcohol flavor has cooked off. Taste and adjust seasoning.
Serve the mushrooms with the sauce drizzled over the top.
Notes
If you don't want to use bourbon in the sauce, you can replace it with vegetable broth.