The Braaaaaaaaaaaaains! mission is a side quest line given by 'Zombie TK Baha'
who can be found far west of 'Hallow's End'. I highly advise you go meet Zombie TK Baha on your way to the waypoint of the third mission 'House of the Ned' so that any zombies you kill throughout the DLC will count.
Now once you've met Zombie TK Baha he will give you the mission quest line, there are 5 missions that all revolve around collecting a set number of zombie brains to give to TK. In order to get zombie brains you need to kill zombies by headshot so grab your best sniper rifle/assault rifle/SMG/Pistol and aim for the head. Every zombie counts and I advise you to return to TK the second you gather enough brains to complete each mission to avoid wasting time grinding at the end.
The 5 missions are:
◾Brains - 10 brains
◾Braaains - 25 brains
◾Braaaaains - 50 brains
◾Braaaaaaaaaaaains - 100 brains
◾Braaaaaaaaaaaaaaaaains - 250 brains