Melt the butter with the dark chocolate and reserve.
Pull apart the egg yolk from the white in two different containers. One of them is for whisk the egg white until stiff o when they form peaks. Then, you can reserve it on the fridge.
Into another container, beat the egg yolk with the sugar until both become to homogeneous mixture.
With a whisk, mix the chocolate blend with the sugar one and add the baker powder (if becomes too roughcast, you can add some drops of water to make it lighter).
Gradually meet the stiff of egg white with the chocolate mixture, until have a creamy mix. You can help yourself by using the hand blender.
Grease the tin (make sure it can work on the microwave) and pour the batter into it.
Cook at microwave at medium power for 6-8 minutes.
