cup unsalted butter, at room temperature
cup grated Parmesan
Kosher salt and freshly ground black pepper
Preheat the broiler to low. In a small bowl, mix the butter, mayonnaise and cheese, then season with salt and pepper. Cut the baguette in half both crossways and horizontally. Evenly spread cheese mixture on the bread. Place on a baking sheet and broil until golden, about 3 minutes.