I will give the recipe that I used first, then how I changed it. I changed it because it was dry and a bit bitter.

Melt 2 tablespoons butter in a mug until melted (about 20-30 seconds),
Add to the mug: 2 tablespoons almond flour, 2 teaspoons coconut flour, 2 tablespoons cocoa powder (not hot chocolate mix!!!), 1 tablespoon of powdered sugar substitute (I used Pyure baking blend), 1/4 teaspoon vanilla extract, 1/2 teaspoon baking powder, 1 large egg. Mix and microwave for 50-60 seconds. This is what I used the first two times I made the cake.

My changes: I used 2 tablespoons of coconut oil instead of butter. The butter I have is salted and I thought it was taking away from the 'sweetness.'
I used two tablespoons plus one teaspoon of almond flour.
I used only 1 teaspoon of coconut flour because it has more carbs than almond flour, that is why I added the additional teaspoon of almond flour to the mix.
Instead of using 2 tablespoons of cocoa powder, I used 1 1/2 tablespoons.
I added an extra teaspoon of baking blend (sugar substitute) to the mix.
I microwaved it for 50 seconds. My microwave is 1000 watts.

I can look up the calories, carbs, etc. if you need them. There are mug cake recipes online that use liquid sugar substitutes if you prefer to use that. The coconut and almond flour are sold at Walmart. They are a bit expensive but they last a while if you are only using a couple teaspoons and tablespoons.