In a microwave-safe bowl, combine the melted unsalted butter, beaten egg, coconut flour, erythritol, baking powder, vanilla extract, and coconut milk. Mix until the batter is smooth and well combined. Pour the batter evenly into two equal-sized ramekins. Microwave the ramekins for 2 minutes, or until the sponge rises and is set. Allow them to cool slightly.

In a mixing bowl, whisk together the mascarpone cheese, heavy cream, and powdered erythritol until the mixture is creamy and smooth. Set this aside.

In a shallow bowl, mix your strong coffee with dark rum. Ensure the coffee is warm for flavor infusion. Adjust sweetness with low-carb options like erythritol.

Once the sponge is cool, slice it into even pieces. Dip each sponge slice into the coffee mixture. Place the soaked sponge layers into serving glasses. Add a layer of the creamy mascarpone mixture over the sponge. Repeat until all ingredients are used, finishing with the creamy layer on top.

For best flavor and texture, chill your assembled Quick Keto Tiramisu in the refrigerator for at least 30 minutes before serving. Dust cocoa powder on top before serving if desired.