Place the butter in a 4-inch ramekin. Melt in the microwave, then allow 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 center comes out clean.
Invert into 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 in a greased skillet, or in the toaster. Butter generously and enjoy!