Preheat the oven to 150C fan forced (300F convection bake) or 160C (325F) in a conventional oven.
Using a 20 cm (8 inch) loose-bottom cake pan with 10 cm (4 inch) sides, line the bottom of the pan with parchment paper. Then line the sides with a strip of baking paper.
Add the butter, chocolate, water and instant coffee powder to a large saucepan over low heat. Melt everything together and gently stir until smooth and melted.
Remove from the heat and pour the mixture into a very large bowl. Let cool slightly.
In a separate bowl whisk together the flour, cocoa powder, sugar, baking soda, baking powder and salt.
Once the chocolate mixture is no longer very hot (it can still be slightly warm), whisk in the eggs. Then stir in the sour cream and vanilla extract until smooth.
Stir the dry ingredients into the wet ingredients. Whisk the mixture until smooth. You can use an electric mixer if you prefer.
Pour the batter into the prepared pan.
Bake in the preheated oven for about 75 - 90 minutes, or until an inserted toothpick comes out clean or with a few damp crumbs. If you gently press the top of the cake with your finger, it shouldn't move. Or if you gently nudge the pan, the cake shouldn't wobble.
Cool the cake in the pan for at least 30 minutes, then turn the cake out and continue cooling on a wire rack.