Pre-heat the oven to 180°C, and line a square baking tin with greaseproof paper.

In a large bowl, whisk together the eggs, sugar, and vanilla until the sugar dissolves.

In a saucepan, melt together the butter and dark chocolate until just melted. Leave this to cool.

Once the chocolate mixture has cooled down to room temperature, whisk it into the sugar and egg mixture.

In a separate bowl, mix together both flours, cocoa, and the chocolate chips, then gently fold this through the wet ingredients until just combined.

Pour the batter into a square tin, and bake for approximately 20 - 25 minutes, or until the middle just begins to set.