Preheat a stovetop pan over medium heat.
Spread butter evenly on one side of each sourdough bread slice.
Place one slice of bread, butter side down, onto the pan.
Layer one slice of cheddar cheese on the bread in the pan.
Add sliced turkey breast evenly over the cheese.
Spread guacamole over the turkey slices.
Top with the second slice of cheddar cheese.
Place the second slice of bread on top, butter side up.
Grill the sandwich for 4 minutes or until the bottom bread is golden brown and cheese starts to melt.
Carefully flip the sandwich using a spatula and grill the other side for another 4 minutes until golden brown and cheese is fully melted.
Remove from heat and let rest for 1 minute.
Cut the sandwich diagonally into two halves.
Serve stacked slightly offset on a white ceramic plate with a small ramekin of extra guacamole on the side.