Heat a skillet over medium heat.
Spread butter on one side of each slice of sourdough bread.
On the unbuttered side of one slice, spread the mashed avocado and season with salt and pepper.
Layer the cooked bacon on top of the avocado, followed by the slices of cheddar cheese.
Place the second slice of bread on top, buttered side up.
Carefully place the sandwich in the heated skillet. Cook for about 3-4 minutes until the bottom is golden brown.
Flip the sandwich and cook for another 3-4 minutes until the other side is golden and the cheese is melted.
Remove from the skillet, let it cool for a minute, then slice in half and serve.