Mix the butter, eggs and brown sugar. Add vanilla and milk. Slowly incorporate the rest of the dry ingredients. Mix well. I prefer using a stand mixer since it's quite thick.
Then, spread 3/4 of the cookie mixture on the bottom of a greased 13x9 pan, pour chocolate mixture, then spread rest of cookie mixture on tops in spoonfuls (won't cover completely-but will leave cool designs). The top can tend to get done before it's completely cooked, so you can put aluminum foil over it for the last ten or so if it looks like it's browning too much. Bake for 25 minutes at 350 degrees. Let cool, otherwise it won't set up and will just run when you cut into it.