Preheat the oven to 350F/177C convection (325F/163C conventional) and grease and line an 8x8” pan with baking spray and parchment paper (leaving an overhang to easily remove them).
Place the butter and chocolate in a medium, heat proof bowl and microwave for 1 minute, remove and stir until the chocolate and butter is melted and combined.
In a large bowl, place the granulated sugar, brown sugar, cocoa powder, vanilla and eggs. Whisk them together aggressively for 2-3 minutes, until the mixture is completely smooth.
Add in the melted chocolate and butter and whisk until the mixture is fully combined.
Add in the flour and salt and whisk just until there’s only a few streaks of flour left.
Spread into the prepared pan and bake for 40-45 minutes. The top should be thin and crackly and puffed in the enter.
Tap it against the counter a couple of times, it should deflate a little but not be jiggly at all. When you press on it, it should feel soft underneath but not liquid-y.
Allow to cool in the pan for 30-60 minutes. Top with a pinch of sea salt flakes and serve warm or cooled.
