Preheat your oven to 350°F. Line a 9x5-inch or 8x4-inch loaf pan with a strip of parchment paper leaving a 2-inch overhang along the long sides and lightly butter the exposed short sides.

Place chocolate and butter in a heatproof bowl and set it over a saucepan with ½-inch of barely simmering water. Stir frequently until melted and smooth. Remove from heat and stir in 2 tablespoons (25g) of sugar followed by the egg yolks and flour.

In the bowl of a stand mixer fitted with the whisk attachment or in a large bowl if using a hand mixer, beat egg whites with salt until soft peaks begin to form. Then, gradually add the remaining 4 tablespoons of sugar, one tablespoon at a time, and beat until the whites are glossy and hold firm peaks.

Use a wide rubber spatula or big balloon whisk to briskly fold one-third of the egg whites into the chocolate mixture to lighten it up. Once incorporated, gently fold in the remaining egg whites just until the ingredients are evenly combined and no white streaks are visible.

Gently spoon the mixture into your prepared loaf pan and bake for 35-40 minutes until the cake feels slightly firm in the center. It will be puffed and cracked on the surface when ready. Transfer pan to a wire rack and let the cake cool completely in the pan. It will collapse and cave in as it cools, but don't worry because that is supposed to happen. Dust generously with cocoa powder before serving. I like to enjoy thick slices of this decadent cake cold from the fridge for a fudgy summer treat, or at room temperature with a dollop of freshly whipped cream.