1Preheat oven to 175°C. Grease and line 20 cm baking tin.
2Melt butter and chocolate together using a bain-marie, ensuring the bottom of the bowl does not touch the water.
3Remove from heat, stir in sugar and salt until dissolved using a wooden spatula instead of a whisk to avoid incorporating air.
4Ensure the mixture is warm, not hot. Add eggs one at a time, whisking between each addition until the batter is completely homogenous and glossy.
5Sift flour and cacao powder into the batter. Fold gently with a spatula until just combined, taking care not to overwork it.
6Pour into a baking tin, tap firmly to remove air bubbles, and finish with extra flaky sea salt. Bake for 22 minutes until the top is set and a skewer comes out slightly moist.