Preheat the oven to 170°C (150°C fan-forced). Grease a 23cm round springform pan and line the base and sides with baking paper.
Dissolve the instant coffee in the boiling water.
In a large heatproof bowl, combine the butter, dark chocolate, and hot coffee mixture. Stir until completely melted, smooth, and glossy.
Add the sugar and whisk until dissolved.
Whisk in the eggs and vanilla extract until fully combined.
Sift together the self-raising flour, cocoa powder, and salt.
Gradually whisk the dry ingredients into the chocolate mixture until smooth. The batter will be quite liquid.
Pour the batter into the prepared pan and gently tap the pan on the counter to remove air bubbles.
Bake for 50–60 minutes or until a skewer inserted into the center comes out clean or with a few dry crumbs attached.
Allow the World's Best Chocolate Cake to cool in the pan for 20 minutes before removing. Transfer to a wire rack and cool completely.