In a small microwave-safe bowl or ramekin (mine measures 3 inches on the bottom and 3.5 inches at the top), melt the butter in the microwave.
Allow the melted butter to slightly cool, and then mix in the salt, egg, coconut flour, and baking powder. Keep mixing patiently until smooth.
Microwave the mixture on high for 90 seconds. The bread will puff up while "baking" and then slightly deflate. When done, it will look like a muffin. Ensure it appears set; otherwise, microwave it for 10-20 more seconds (but 90 seconds should be enough, and overcooking can dry it). Take care when removing it from the microwave. The bowl will be hot.
Let the bread slightly cool for a minute or so. Then, gently, with a paring knife, loosen the edges and remove the bread from the bowl onto a plate.
Slice the bread crosswise into two slices and top it with your favorite toppings.
To toast the bread:
Heat a pat of butter in a medium skillet over medium-low (not higher) heat. Or spray the skillet with olive oil spray.
Add the bread slices. Cook, gently pressing on them with a spatula to ensure even toasting, until golden, about 3 minutes per side.
Video
Notes
To make this bread in the oven, mix the batter in an oven-safe ramekin. Bake the bread in an oven preheated to 350°F for about 10 minutes. Check with a toothpick to ensure it's fully baked. Ovens vary, so keep an eye on the bread. It might take a shorter or longer time to be fully baked.
Toasting the bread in a buttered skillet greatly improves its flavor and texture, but this is not mandatory. Usually, I don't bother. However, if you try this bread and find that you dislike it (some do), I highly suggest trying it toasted.
To keep the leftovers, slice each piece of bread crosswise into two slices and allow the slices to cool completely. Then, place them in an airtight container in the fridge for up to 3 days.
When ready to use, I highly suggest skillet toasting the leftover slices in butter or in olive oil. But you can also gently reheat them in the microwave, about 10 seconds per slice, at 50% power.
You can also freeze slices of the completely cooled bread. Microwave them frozen to defrost, or use your toaster on the lowest setting.