In a large bowl, sift together flour, cocoa powder, baking soda, and salt. Mix in granulated sugar. Set aside.
To make the chocolate ganache: place chocolate and heavy cream in a medium heat-proof bowl, and microwave in 20-30 second-intervals, stirring in between each interval, until melted. Alternatively, set the bowl over a saucepan of simmering water, stirring occasionally, until melted. Pour evenly over the cake. Refrigerate for 1 hour until the ganache is set.