Place a large skillet over medium-low heat and melt 3 tablespoons butter. Add 4 slices bread and toast on each side for 2 to 3 minutes or until each slice is golden brown.
Remove from heat and set aside.
Using a round, 2 inch cookie cutter cut a hole out of the center of the remaining slices of bread.
Melt 2 tablespoons butter and toast each slice of bread for about 2 minutes. Flip each slice and place 1/2 tablespoon butter in the hole of each slice. Carefully drop a yolk into each buttered hole and continue to toast for about 2 minutes. Remove from heat and set aside.
To assemble: Spread mayonnaise onto the inner sides of each of the four plainly toasted bread slices. Top two of the slices with turkey followed by tomatoes and top with a yolk centered slice of bread. Top second piece of bread with butter lettuce followed by crispy pancetta, (slice avocado, if using) and finally the remaining bread slices. Serve.