Preheat oven to 385 degrees F. Grease a round cake pan, line with parchment paper (round) and grease the parchment paper. Parchment paper will help the cake seamlessly release from the pan.
Cut the butter into pieces so it melts evenly. Place in a large, heat-proof bowl. Add chopped chocolate. Melt in 20 second increments in the microwave, stirring after each session until completely melted and smooth. Let cool for 2-3 minutes. You can use a double boiler for this step, if desired.
Whisk the sugar, coffee powder, and vanilla extract into the chocolate mixture. Whisk in the eggs, cocoa powder and salt until smooth. The mixture will be heavy and tacky, like ak thick brownie batter.
Pour and spread the batter into the prepared cake pan.
Bake for 30 minutes, or until the edges are set. The cake is done when a toothpick inserted in the center of the cake comes out clean or, comes out with a few moist crumbs. It's important not to over-bake this cake, which will quickly dry it out. Begin checking at 25 minutes. Don't be alarmed if the cake rises extra tall around the edges and slightly cracks - this is normal (it's the eggs expanding) and will flatten out as it cools.
Remove from the oven and place the cake on a cooling rack. Cool for only 10 minutes in the pan. Run a sharp knife around the outer edges of the pan to release the warm cake, then quickly invert it onto a serving plate or cake stand (If it cools completely in the pan it will be very difficult to release). Cake will be a little crumbly on the edges. Cool completely. Put in the refrigerator 1-2 hours.
Once the cake is cool, top with your choice of toppings.