Preheat the oven to 175°C (350°F) fan.
Melt the chocolate and butter together in a small pot on low heat.
Whisk the egg, light brown sugar, cane sugar, and sea salt in a medium-sized bowl for 4 minutes on medium speed with a handheld mixer until light and fluffy.
Pour the melted chocolate and butter mixture into the egg mixture and stir for 1 minute.
Fold in the flour and cocoa. Don't overmix.
Scoop the dough immediately onto a prepared baking sheet with a medium-sized cookie scoop (about 1.5 tablespoons). I baked all 12 on one sheet.
Bake for 7-8 minutes until the top is glossy and the edges are set. Do not overbake.
Reshape the cookies carefully with a glass or spoon if they aren't completely round.
Top with flaky sea salt.
Allow to cool on the baking sheet for 5 minutes.
Transfer to a cooling rack.