Preheat the oven to 300°F/150℃. Prepare a 9-inch cake pan by lining the bottom with parchment and lightly buttering the sides. If desired, dust with cocoa powder and tap out any excess.
Melt the chocolate in a medium-sized microwave-safe bowl in the microwave for about 30 seconds. Then add butter and continue melting in 30-second zaps, stirring after each interval, until completely melted and smooth. This might take a couple of minutes. (If you don't like using microwaves, you can melt them in a double boiler over medium heat, stirring occasionally.) Set aside.
Whisk the eggs in a large bowl with a mixer for about a minute. Add sugar, vanilla extract, and salt, then continue mixing until it's pale and frothy.
Pour in the chocolate and butter mixture, and mix well.
Finally, add the cocoa powder and continue mixing until fully incorporated. Be careful not to overmix.
Pour your cake batter into the prepared pan, and then bake it for 40-45 minutes (a long toothpick or skewer inserted into the cake will come out moist, but no clumps, an instant-read thermometer will say 200℉/95℃, and it should be moist but not wet.)
Remove the chocolate cake from the oven and allow it cool while preparing the raspberry sauce.
As the cake cools, it will sink in the center, but don't be alarmed. That's the beauty of this cake.
Cover the cooled cake still in the pan with plastic wrap and move it to the fridge. Then let it chill for five hours or overnight.
When ready to serve, run your knife around the edge of the cake pan to loosen the cake. Remove the cake from the pan and trim the raised rim around the cake's edge if necessary.