Add butter and 'nduja to a skillet, then heat until melted. Use a wooden spoon to help break up the 'nduja. Once melted, add garlic, then cook an additional 30 seconds.

Turn the heat up. Add maitake mushrooms to the pan and cook, stirring often, until browned and tender, about 3-4 minutes. Season with salt, then transfer to a serving bowl.

Make a well in the middle of the plate and immediately place the egg yolk in the well. Use a fork to break the yolk, then mix everything together to coat. Serve immediately.