Preheat oven to 450°F.
Place 2 tablespoons butter on a cookie sheet. Place cookie sheet in the oven until butter has melted.
Once butter has melted, remove cookie sheet from the oven and spread melted butter over the entire surface of the cookie sheet using a basting brush.
Top pan with 6 tortillas, overlapping them and leaving half of each tortilla hanging over the edge of the pan. Set aside.
Place avocado, cilantro, sour cream, lime juice, garlic powder, salt and pepper in the bowl of a food processor.
Process until smooth. Taste and season with additional salt if needed.
Place shredded chicken in a large mixing bowl.
Add sauce and stir to combine. Set aside.
Spread out ½ the grated Jack cheese onto the tortillas.
Top with reserved/sauced chicken meat in an even layer.
Top with remaining cheese.
Fold tortillas over towards the center forming an envelope to enclose the cheese and chicken.
Brush the tortillas with melted butter and top with another cookie sheet.
Bake for 20-25 minutes until tortillas are lightly browned, the filling is hot, and the cheese is melted.
Remove from the oven and transfer onto a cutting board.
Cut into squares and top each serving with a dollop of sour cream to serve.