Melt 1 tablespoon of butter in a large skillet over medium-high heat. Stir in garlic, and cook for 30 seconds until fragrant. Add mushrooms, and continue cooking until softened and lightly browned, 4 to 6 minutes. Season with salt and pepper, then set aside to cool.
Meanwhile, spread softened butter onto one side of each slice of bread. Press the buttered sides into a muffin tin; set aside.
Stir together eggs and cream, then stir in the cooled mushroom mixture. Divide this custard equally among the toast cups. Sprinkle the tops with grated Parmesan cheese.
Bake in preheated oven until egg mixture sets, and tops are golden brown, about 20 minutes.