I've made many versions of the cake over the years. It's very flexible, especially with regard to the solids used (almond flour, other nut flour, flax meal, more cocoa, protein powder, oat fiber, etc.).

Here's one of my favorite versions, which is relatively low in both net carbs and calories, and has extra chocolate.

Ingredients:

1 extra large egg
10 drops Sweetzfree (liquid splenda) or sweetener of choice
3 T sour cream
1/2 t baking powder
3 T cocoa powder
2 T ground flax (flax meal)
1 T oat fiber

In a 2 cup Pyrex or other container, blend the ingredients in the order listed, mixing by hand after each addition. Microwave on high for about 80 seconds. The cake should look a slightly wet when it comes from the microwave; it will dry as it cools. This makes 2 servings.

Nutritional Info for the entire 2-serving cake, assuming the cocoa that you're using as 1 g net carb per tablespoon (I use Dagoba brand):

Calories - 275
Total Fat - 23 g
Saturated Fat - 7 g
Polyunsaturated Fat - 1 g
Monounsaturated Fat - 5 g
Cholesterol - 264 mg
Sodium - 295 mg
Potassium - 139 mg
Carbohyrdate - 21 g (total)
Fiber 14 g
ECC = net carbs 7 g
Protein - 15 g
Vitamin A - 11%
Vitamin C - 1%
Calcium - 27%
Iron - 13%

ECC (net carb count) for one serving is 3.5 g

Note that "oat fiber" is not the same as "oat bran." Oat fiber is fiber only and has 0 calories and 0 net carbs. If you don't have oat fiber, it can be replaced with another tablespoon of flax meal or almond flour or protein powder, etc. The solid part of this is very flexible.

If you want to cut the calories further, you could use low fat or non-fat sour cream, although that would raise the carb count slightly.