Put the coconut oil in a medium bowl. (If it's solid, microwave if first for a few seconds to melt.) Whisk in the eggs, maple syrup or agave and vanilla until smooth. Add the cocoa powder and salt and whisk again until completely smooth. Stir in the nuts and chocolate chips.
Scrape the batter into a 12-ounce (or larger) ceramic coffee mug. Microwave on high until the cake is cooked through and peeks over the rim of the mug, about 80 seconds. (Every microwave is different, so watch it as it cooks.). Top with some fruit if you'd like and serve right away.