In a medium bowl, combine the chicken, cheeses, salsa, and Greek yogurt.
Spread about ½ Tbsp butter on the outside of each slice of bread.
Divide the mixture evenly between four of the bread slices, and top with another slice of bread.
Heat olive oil in a pan over medium-low heat.
Cook the sandwiches for about 4-5 minutes per side, or until golden brown and the cheese has melted. Covering the pan with a lid during cooking will help the inside get all warm and gooey before the bread gets too browned.