Place the butter in a microwave-safe 4-inch ramekin or mug. Melt it in the microwave, then allow it to slightly cool.
With a fork, mix in the egg and salt.
Mix in the almond flour, thoroughly mixing until smooth. Finally, mix in the baking powder.
Microwave for 90 seconds, until puffed and a toothpick inserted in the center comes out clean.
Invert the muffin onto a cutting board and allow the bottom to air out for a couple of minutes. It will be very moist. If the bottom sticks to the ramekin and the muffin won't come out, carefully loosen it with a spoon.
Cut the muffin crosswise into two slices.
Toast the slices in a greased skillet or use the toaster on the lowest setting.
Butter generously and enjoy!
Video
Notes
The ramekin I use holds about ¾ cup (6 ounces) of liquid. But you want it to be wide and shallow, not small and deep. The diameter is important here.
For the best results, toast the muffins on both sides in a buttered skillet. However, I often use a toaster on the lowest setting, and the results are fine, though not as good as skillet toasting.
The leftovers keep well in the fridge for 3-4 days. Not that I often have leftovers! But sometimes, I like to make a few of these muffins the night before and reheat them in the morning. So I microwave the muffins, toast them, cool them completely, and store them in a Ziploc bag in the fridge. The next morning, I briefly reheat them in the microwave, and they are perfect.
You can also freeze the cooled muffins for up to 3 months. If stacking them, place wax paper between the layers.