Place the butter in a microwave-safe wide mug or shallow ramekin and microwave for 20–30 seconds, until melted. Swirl the butter around the base and sides to grease the container.

Add the egg to the melted butter and whisk thoroughly until the mixture is evenly combined. A well-beaten egg helps create a cleaner, more uniform texture.

Stir in the almond flour, coconut flour, baking powder, and optional salt. Scrape the bottom and sides as you mix, then let the batter stand for 30–60 seconds so the coconut flour can absorb moisture.

Microwave on high for approximately 90 seconds. Begin checking around 75–90 seconds because microwave strength varies, and stop when the center looks firm and set.

Let the bread stand for about 30 seconds before loosening the edges with a knife. Remove it from the ramekin, slice it in half horizontally, and serve warm or toast the slices.