Add 1 cup mayonnaise, ¼ cup white sugar, 4 teaspoons white vinegar, ¼ teaspoon dry mustard, and ¼ teaspoon kosher salt to large mixing bowl. Whisk ingredients together until fully incorporated.
Add 2 14-ounce bags shredded coleslaw mix to bowl. Using large silicone spatula, gently fold coleslaw mix into prepared dressing, stirring until coleslaw is completely coated.
Cover bowl with lid or plastic wrap and place in refrigerator. Chill coleslaw at least 2 hours.
For the BBQ Chicken
Place 2 large boneless, skinless chicken breasts in Crockpot.
Add ½ cup BBQ sauce, 2 tablespoons Italian dressing, 1 packed tablespoon brown sugar, 1 teaspoon Worcestershire sauce, and 1 teaspoon salt to mixing bowl. Stir ingredients together until fully combined. Pour prepared BBQ sauce mixture over chicken in Crockpot, covering chicken breasts completely.
Secure lid on Crockpot and set time and temperature to 3 to 4 hours on HIGH heat. Flip chicken breasts over occasionally as needed to keep chicken covered in sauce.
After 3 hours, check internal temperature of chicken breasts with internal meat thermometer. Insert thermometer into thickest area of chicken breasts. Remove chicken from Crockpot once chicken reaches 160° to 162° Fahrenheit.
Transfer cooked chicken to cutting board or large bowl. Let chicken rest 3 to 5 minutes, then use 2 forks or hand mixer to shred chicken to desired size pieces.
Return chicken to Crockpot. Gently stir chicken into BBQ sauce, making sure to cover shredded chicken completely.
Set heat to LOW heat. Simmer BBQ chicken 15 to 30 minutes, or until some sauce has been absorbed.
For the Toasted Buns (Optional)
Place large skillet on stovetop over medium heat. When skillet is warm, add 2 tablespoons unsalted butter. Let butter melt completely, swirling and tilting pan occasionally to move butter across entire surface.
When butter is melted, place tops from 6 brioche buns in skillet, sliced-side down. Toast inside of brioche bun tops 1 to 2 minutes or until just golden and crisp. Transfer tops of buns to plate and set aside.
Add remaining 2 tablespoons unsalted butter to large skillet. Again let butter melt completely, swirling and tilting pan occasionally to move butter across entire surface.
When butter is melted, place bottoms from 6 brioche buns in skillet, sliced-side down. Toast inside of brioche bun bottoms 1 to 2 minutes or until just golden and crisp. Transfer bottoms of buns to serving plate(s) and place toasted-side up.
To Serve
Remove prepared coleslaw from refrigerator. Stir well to reincorporate ingredients.
Place large scoop of BBQ chicken on toasted-side of each brioche bun bottom. Gently flatten scoop of chicken slightly, then place dill pickle chips on top of chicken if desired. Top pickle chips with scoop of chilled coleslaw and cover with toasted top bun. Repeat with remaining sandwiches, then serve immediately with additional BBQ sauce if desired.
Recipe Notes
Crockpot: I used a 6qt. Crockpot for this recipe. If your slow cooker is larger, you may need to increase the amount of BBQ sauce you use in order to keep the chicken fully covered during cooking. Any exposed areas of chicken can dry out.
BBQ Sauce: Any brand and any flavor BBQ sauce will work. You can also make our keto BBQ sauce or Whole30 BBQ sauce if you'd rather!