Heat the oven to 180 ° C. Vystelit paper square 20 x 20 cm or to take a large silicone cake pan with a hole. The dough rises strongly, the form of need a large. Sift the flour with the baking powder. Set aside. Mix cocoa powder with coffee and pour 50 ml of cold water. Stir. Beat the butter with the sugar, add the cocoa mixture with coffee, and stir. To drive eggs one by one. Melt in the micro or in a water bath chocolate, stir into the dough. Put the sour cream and whisk. Add the flour and beat well. Put the dough in shape and bake until dry splinter about 50 minutes. The cake to cool and sprinkle with powdered sugar.

To serve, cut into pieces.