Set up the double boiler on the stovetop over low heat.

Preheat the oven to 150° C (300° F).

Chop the dark chocolate into pieces and place in the top of the double boiler with the butter. Stir with a wooden spoon until the mixture is completely melted.

Mix in the sugar, then the eggs. 

In a bowl, combine the flour, powdered cocoa and baking powder.
 

Add these dry ingredients to the chocolate mixture in the double boiler, sprinkling in gradually and stirring until the two are smoothly combined.
 

Take a round cake pan, approximately 22 cm (9") in diameter.
 

Using a brush or a small cloth, butter (not excessively) the inside of the pan.

Place two big spoonfuls of flour into the pan and turn it around with your hands. The flour should spread out and coat all the inside surfaces of the pan. Dump out the excess flour from the cake pan.

Pour the batter into the pan and bake in the oven for 15 minutes.
 

Unmould the cake onto a serving platter and serve while still warm.

To be successful the cake should remain slightly runny in the middle, which makes it very soft and moist.

You could serve a vanilla sauce on the side.