Preheat the oven to 190 C and line an 8 inch springform pan with non-stick baking paper. I find just greasing a pan for this recipe isn't enough, the cake tends to stick, so paper is highly recommended. You can use a regular 8 inch round cake tin too, but springform is easier to slice and remove the cake.
Melt the chocolate and butter in a microwave-safe mixing bowl, in ten second increments, stirring in between each. Do not let the chocolate get too hot, instead stir to melt down any lumps. Alternately, use a double boiler or bowl set over a saucepan of simmering water for this step, then pour the mixture into a mixing bowl.
Into the melted chocolate and butter, add the sugar and whisk well to combine. A balloon whisk is useful in this recipe. Add in the orange zest and vanilla extract and whisk again.
Add the eggs, one at a time, whisking to incorporate. Finally, sift in the cocoa and gently fold the batter using a silicone spatula to combine till smooth and thick.
Pour the batter into the prepared dish and bake for 20 to 25 mins or until a toothpick poked in the middle comes out with only a few moist crumbs. The top will have developed a crust. Over-baking will lead to a dry cake. Allow to cool for 30 minutes.
Best eaten freshly baked, when still slightly warm and with vanilla ice cream! The crust will lose its crispness once the cake is covered, but it's still delicious of course. Store in an airtight tin at room temperature for 2 to 3 days, and in warm weather, transfer straight to the fridge once it's completely cooled. Reheat individual pieces as needed. Happy baking!