Preheat oven to 160°C. Generously grease an square or round tin, then line the base with parchment.

Gently melt the butter and chocolate together, either over a pan of simmering water or in the microwave. Stir until smooth and set aside.

In an electric mixer, over medium speed, beat sugar and eggs, one at a time, beating well after each addition. Add vanilla extract and continue beating until fluffy.

Using electric hand beaters, whisk the egg whites, and a pinch of salt until stiff but not dry. Set aside.

Pour the chocolate mix around the edge of the bowl with egg yolks and sugar mix, then fold together using a large wooden spoon until the batter is well combined.

The brownies may sink and crack a little, which is fine. Carefully cut into squares. Serve with whipping cream or ice cream, if desired.