I tried a lot of recipes for "cake in a mug" but none of them were that good. My brainstorm was I took my favorite chocolate cake recipe and made a smaller version to fit in a mug and it turned out great! This is good when you crave something sweet but don't want to make an entire recipe of something. (Note: this recipe does NOT call for an egg.)
Put all the ingredients EXCEPT 1 TABLESPOON OF THE WATER AND THE NUTS in a small bowl. Whisk about a half minute until well combined then whisk in the remaining 1 tablespoon of water. (Use a small whisk to ensure all is thoroughly combined.) Stir in the nuts.
Pour into a large ungreased (microwaveable--I use ceramic) coffee mug. Nuke on high for 1 minute and 15 seconds. Test with a toothpick. If it doesn't come out clean, nuke another few seconds or so (My microwave is 1350 watts---cooking time will vary with the wattage of your microwave.)
Don't overbake it. It should be nice and fudgy. Let sit a few minutes before you dive in. This is good as it is but if you want to 'gild the lily', you can drizzle it with a little chocolate syrup.