Prepare boxed brownie mix according to package directions. Pour into 4 mugs (mugs should be 1/2 to 3/4 full). Place mugs in crockpot. Cover crockpot and cook on high for 1.5-2.5 hours. You will know they are done when they have risen, pulled away from the sides of the mugs and a knife inserted into the middle is comes out clean. Top with whipping cream or ice cream.
PS- My brownie mix was for an 8x8 pan and it only filled up two mugs. Mark and I demolished them. It didn't seem like I was eating 1/2 a pan of brownies when it was housed in a cute mug. Dangerous.