Combine all the buttermilk marinade ingredients in a large bowl then add the chicken breasts. Allow to marinade for at least 2 hours but up to 24 hours, refrigerated.
When the chicken has marinated, combine all the ingredients for the flour dredge and mix well.
Remove the chicken from the buttermilk marinade then dredge in the flour, making sure to get the flour packed onto the chicken.
Heat oil in a deep pot. Shake off the excess flour then carefully add the chicken to the hot oil. Cook for 8-10 minutes until the chicken is golden brown and crisp and cooked through. Flip half-way through cooking to allow both sides to brown.
Remove the chicken from the oil and allow to drain before assembling the sandwich.
In the meanwhile, combine all the ingredients for the coleslaw in a bowl and mix well.
Toast the brioche rolls then top with the chicken and coleslaw and serve.