Preheat your oven to 350°F/180°C. Butter the sides of a 9x5-inch loaf pan and line the bottom with a strip of parchment paper.
In a heatproof bowl set over a saucepan of simmering water, melt the chocolate and butter together until smooth.
Remove from heat and stir in 3 tablespoons of the sugar, the egg yolks and flour.
Using a mixer, beat the egg whites and salt until soft peaks begin to form. Gradually beat in the remaining 3 tablespoons of sugar and whip until the whites are smooth, glossy and hold firm peaks.
Use a wide rubber spatula to fold 1/3 of the egg whites until the chocolate mixture. Once incorporated fold in the remaining egg whites in two parts just until the mixture is smooth and no white streaks are visible.
Scrape the batter into the prepared pan and bake for 30-35 minutes, or until the cake has puffed up and feels slightly firm in the center. Do not over-bake. Let the cake cool in the pan. It will collapse as it cools as it should. Let it cool to room temperature before serving.