Lightly spread one side of all bread slices with butter.
Place one slice of bread on a plate, butterside down, and cover with 1 slice of cheddar, half of the avocado slices ( I mash them down a bit) and finally, a slice of gruyere or provolone cheese.
Top with the remaining piece of bread, butterside up.
Preheat a panini maker or saute pan over medium heat and cook the sandwiches until the bread is golden and the cheese is melted, about 4 minutes.