Preheat oven to 180 degrees Celsius
Lightly grease a 23 x 23cm square baking pan and line with parchment paper, you will need the paper slighly overlapping the pan for easy removal
Place the chocolate chips and butter in a microwave safe bowl and melt for 50 seconds or more if required. Set aside
Place the eggs, vanilla essence and sugar in a bowl and using an electric mixer beat until it doubles in volume, for about 3 minutes, on a medium speed
Slowly drizzle in the chocolate mixture and keep beating until everything is incorporated, about 10 seconds
Sprinkle the sifted flour on top and beat for another 8 seconds. Add the chopped chocolate and use a spatula and give it a quick mix
Pour the batter into the prepared pan. Bake for 30-35 minutes until it's just set. If a toothpick is inserted it should come out a little dirty. It will continue cooking in the hot pan once it is out of the oven.
Remove and cool in the pan until it reaches room temperature, about 30 minutes . Cut into 16 pieces.