Preheat the oven to 325 degrees F. Spray a springform pan with cooking spray and line the bottom with a circle of parchment. Coat the top of the paper with the spray.
Melt the chocolate and butter in a double boiler, stirring in 45-second intervals to avoid burning. Set aside to cool for several minutes.
In the bowl of a mixer, beat the granulated sugar, brown sugar and eggs until fluffy. With the mixer on low, add the cocoa powder, vanilla and salt. Slowly drizzle in the cooled chocolate mixture and mix until just combined.
Pour the batter into the pan and smooth the top. Rap the pan on the counter a couple of times to release any trapped air bubbles. Bake until just set on top, about 50 minutes. Remove and allow to cool completely, then chill in the refrigerator for at least 2 hours or overnight.
Just before serving, remove the cake from the pan and place it on a platter. Sprinkle the top with sifted powdered sugar.