Add the coconut oil to a 12 oz (355 ml) microwave-safe mug; microwave 10 seconds, or until the oil is melted. Rotate the mug around to coat the entire inside with the oil.
Whisk the egg in a small bowl, then whisk in the honey, almond milk, and vanilla.
Stir in the almond meal, cocoa powder, flaxseed meal, salt, baking soda, and espresso powder until smooth.
Pour off any extra coconut oil from the mug into the batter and stir it in.
Pour the batter into the prepared mug and microwave on high until cooked, about 2 minutes in an 800W microwave. (If your microwave has a higher wattage than that, start with 1 minute and add 10 second intervals until it’s done.)
Cool slightly and then serve immediately (it’s best eaten when still hot).