Add cut butter and finely chopped chocolate to a large heat proof bowl. Microwave for 45 seconds, remove from the microwave and stir. Repeat the process until melted and smooth. Alternatively, melt in a double boiler over low heat.
Allow the chocolate to cool slightly, then mix in the sugar.
In a separate small bowl, mix together the eggs and additional egg yolk. Then whisk the eggs, cognac and vanilla into the chocolate mixture.
Add the cocoa, instant coffee powder and salt into the chocolate mixture. Whisk together until just combined. Be careful not to over mix.
Pour the batter into the prepared pan, lined with parchment paper.Bake at 375 degrees for about 25-30 minutes, or until the top is set and an inserted toothpick comes out clean.
Cool the cake completely before carefully removing it from the pan and out of the parchment paper.
Heat the cream until almost boiling – either in a double boiler over low heat or in the microwave in 30-second intervals.
Pour the hot cream over the chopped chocolate. Let it sit for 3-5 minutes, then whisk until smooth. If the chocolate hasn’t completely melted, microwave for 30 seconds, then remove from the microwave and whisk until smooth. Repeat the process if needed.
Pour the chocolate ganache on top of the cake and spread with a flat spatula or a knife.
Decorate as desired or leave it plain.