In a small bowl, mash the banana and the almond butter together with fork until smooth. Whisk in the egg and the maple syrup.
Add the coconut flour, baking powder, cinnamon, and salt. Stir to fully incorporate. Mix in any of the optional add-ins; a tablespoon of one or two of them is a good starting point.
Scoop into a ramekin or bowl–no need to grease. (I use a dish with a diameter of 4 inches and a depth of 2 inches. You may vary the dimension but may have to alter the cooking time up or down accordingly–and to avoid overflow, don’t go much smaller.) Microwave on high (100 percent power) for 2 to 2 1/2 minutes. Until you know how long this takes in your microwave, check it about 20 seconds early and then add time in 15-20 second increments until the center doesn’t feel squishy when lightly pressed.
Allow the cake to cool for a few minutes and then loosen it with a dinner knife and pop it out of the ramekin or bowl. If not eating right away, place on a cooling rack to prevent the steam form making the bottom damp. The cake may be made in advance, cooled, covered and refrigerated.