If preparing raw chicken: Place chicken in a 6 quart stockpot of boiling water and cook for 10-12 minutes, or until insides are no longer pink. Place in a large bowl to let cool, then use two forks to shred into pieces.
Preheat oven to 375 degrees F. Spray a 9x13 baking dish with cooking spray, then set aside.
In a large skillet, melt butter over medium heat. Add shredded chicken and toss to coat. Cook for 5-7 minutes or until butter is absorbed and chicken is fragrant.
Add beans, corn, and diced tomatoes to skillet. Pour taco seasoning in 1 cup water, stir, then drizzle on top of other ingredients. Toss all ingredients together until blended. Reduce heat to low and cover while preparing the next steps.
For the Cheesy Bisquick Topping
In a medium bowl, whisk together eggs and sour cream. Add Bisquick and whisk until there are no more clumps, then fold in 2 cups of cheddar cheese.
Putting it All Together
Pour chicken taco mix in the bottom of the baking dish, spreading out into an even layer.
Add Bisquick topping 1/4 cup at a time, spreading with a spatula so that the topping completely covers the chicken taco mix.
Bake uncovered for 30 minutes.
Remove from oven. Top with crushed tortilla chips and sprinkle with remaining 1 cup cheddar cheese. Cook for another 5 minutes or until cheese has melted. Let casserole cool 10-15 minutes before serving.