Place the ritz crackers in a ziploc bag. Use a rolling pin to crush the crackers till fine.
In a small bowl, mix the crushed crackers and melted butter together till combined.
Place half of the cracker mixture in the bottom of a 9x13" pan. Set aside.
In a medium sized bowl, mix the cooked and cubed chicken, cream of chicken soup, sour cream and poppy seeds till combined.
Pour the chicken mixture over the crackers. Top with the other half of the cracker mixture.
Bake for 30-40 minutes.
Notes
I have made this in a 11x7 " dish with using only 1 cup of sour cream, 6 chicken tenders (which is about 3 chicken breasts) and kept everything else the same. Sometimes I will add a dash of worcestershire sauce to the cream mixture for fun ;).
Recipe by Baked in AZ at http://bakedinaz.com/poppy-seed-chicken-casserole/