Not sure where that carb count is coming from. I get:

Butter = 0
Splenda 2 C = 48 carbs
2 tsp vanilla = 1 carb
4 eggs = 2.4 carbs
Unsweetened cocoa = 0.4 carbs/tsp, 3/4C = 37 tsp, so 15 carbs
I cup Atkins bake mix = 12 carbs
-----------------
Total of 78 carbs

Topping:

Cream cheese 8 oz = 6
1 cup heavy cream = 7.5
1/2 C Splenda = 12
1/2 C PB = 17
-----------------
Total of 43 carbs


So a grand total of 121 g of carbs. Let's say you get 24 brownies in a 9x13 pan, that's 5 carbs per brownie.