Prepare guacamole by mashing the ripe avocado in a bowl with lime juice, salt, and pepper until smooth but slightly chunky.

Spread guacamole generously on one slice of bread, then layer cheddar cheese on top. Place the second slice of bread over it.

Butter both outer sides of the sandwich evenly.

Preheat a skillet over medium heat and place the sandwich in the skillet; grill for about 3-4 minutes until golden brown.

Flip carefully using a spatula and cook until the other side is crispy and cheese melts, about another 3-4 minutes.

Remove from heat, slice diagonally, and serve warm with extra guacamole or salsa.