Preheat oven to 325F (160C) and line the bottom of an 8” springform pan with parchment paper.
Combine butter and chocolate in a medium-sized heatproof bowl and microwave in 15 second intervals (stirring well in between) until mixture is melted and smooth. Stir in instant coffee and vanilla extract and set aside.
In a separate large bowl (or in the bowl of a stand mixer fitted with a whisk attachment) combine eggs, sugar, and salt. Beat on high speed until mixture has doubled in volume (this will take about 5 minutes with a hand mixer or 1-2 minutes with a stand mixer).
With mixer on low speed, slowly drizzle melted chocolate mixture into egg mixture. Once all chocolate has been added, increase mixer speed and stir until completely combined.
Sift cocoa powder over batter and use a spatula to fold completely into batter.
Pour batter into prepared springform pan and transfer to 325F (160C) oven and bake for 30-35 minutes. Edges should look baked but center may seem jiggly still.
Allow to cool completely at room temperature then transfer to refrigerator to chill for another 6 hours.
If desired, before serving top with chocolate ganache that has been allowed to cool for at least 15 minutes after preparing.
To slice, run a knife under very hot water then dry off the knife and use hot blade to slice cake. In addition to the ganache, enjoy topped with berries and whipped cream, if desired.