In a small bowl mix together the cacao powder, Swerve, salt, baking powder, and optionally espresso powder.
Add in the remaining wet ingredients and mix until smooth.
Grease a small ramekin and pour all but 2 tablespoons of the batter into the ramekin.
Place the piece of dark chocolate in the middle and spoon the remaining batter over it to cover.
Microwave for 1 minute just until the outside is set, it should still wiggle in the middle significantly.
If the edges arent set cook in 15 second intervals until set.
Preheat the oven to 175 °C/ 350 °F and cook for about 12-15 minutes or until cooked in the centre.
Let stand for 1-2 minutes before flipping onto a plate to serve.
Serve immediately.