Preheat oven prior to 175 C. In a square pan (20x20 cm) put a large piece of aluminum foil or baking paper, leaving him to go outside the walls with 5 cm. Lightly oiled fthe oil 1 tbsp butter. Break the natural and bittersweet chocolate into small pieces. Cut 3/4 cup butter in small lumps. Melt the chocolate and the butter in a water bath. Leave to cool for 10 minutes, stirring often. Take the yolk of one egg and put it in a large bowl. Add the yolk to the other 2 eggs. Beat with a mixer. Add the sugar. Crush until the mixture turns white and thickened. Add vanilla. Sift flour, cocoa and salt. Add the melted chocolate to the mixture of eggs and sugar. Mix well. Three times, add the mixture of flour and cocoa, stirring well after each addition. Pour the dough into the prepared baking pan. Bake 35 minutes, while checking with a stick in the center, the match came with a few moist crumbs on it. Remove from oven and place on BBQ, to cool completely. Remove the cake by pulling the ends of the aluminum foil. Place on cutting board. Cut with a knife 4 in each direction. Dip the knife in hot water and dry between each cut. Serve the cookies topped with ice cream and a cup of cold milk or hot coffee.