Set the oven to 165°C/325°F and line your baking tray with parchment paper. Melt the butter, vanilla, and chocolate together in a bowl over some hot water (double boiler). Stir frequently and once completely melted turn off the heat but keep warm.
2. Step
In a separate bowl, beat the eggs and salt together vigorously until frothy. Then, add ½ of the sugar and blend until dissolved. Repeat until all of the sugar is dissolved. Once all of the sugar is dissolved continue beating until the eggs double in size and turn pale. This takes about 8-10 minutes and should be done on medium speed using a stand mixer or hand mixer.
3. Step
Once the eggs are pale and yellow, slowly add in the melted chocolate, butter and vanilla mixture and continue to mix with the mixer on low speed.
4. Step
Next, stir in the flour. Once nicely mixed, pour the mixture into the lined baking tray.
5. Step
Bake for 35-45 minutes at 165°C/325°F until the centre comes out clean when poked with a toothpick.
6. Step
Let cool on a resting rack for 20 minutes, cut, and serve.
