Preheat oven to 425 degrees.
Cook chicken in a separate pan until done. Let chicken cool a few minutes and shred with a mixer.
Using a small saucepan, melt butter on low heat, whisk in flour and add salt and pepper to taste.
Whisk the milk in the saucepan mixture slowly pouring a little bit at a time. Mix until you get a creamy consistency and add in ranch dressing and mix.
Spray a 9×13 baking dish and add a layer of chicken, then sauce, then half of cheese, then tater tots and cook for 20 minutes uncovered.
Crumble cooked bacon while waiting for the casserole to cook and set aside.
Make sure the tater tots are a golden color and crispy and then top the casserole with the remaining cheese and crumbled bacon. Bake for another 5 minutes or until cheese is melted and bubbly.
Take out of the oven and let cool for a few minutes and then enjoy!