Preheat the oven at 320°F (160°C).

Line a 9x9-inch/24x24 cm square baking pan with parchment paper, leaving some overhang for easy removal. Lightly grease with nonstick spray and set aside.

Place the dark chocolate and butter in a heatproof bowl set over a saucepan of gently simmering water (double boiler method). Stir until completely melted and smooth. Remove the bowl from the heat.

Whisk the sugar and eggs into the warm chocolate mixture until fully incorporated and slightly thickened. Add the flour and salt, stirring just until the batter is smooth and uniform.

Scrape the batter into the prepared pan and smooth the top with a spatula. Bake using the convection (fan) setting for 12 to 15 minutes. The edges should be set, while the center remains slightly jiggly.

Let the brownies cool at room temperature for 5 minutes, then transfer the pan to the freezer and chill for 1 hour, until firm.

Heat the cream in a small saucepan or in the microwave until just beginning to boil. Pour the hot cream over the chopped dark and milk chocolate in a medium bowl. Let sit for 1 minute, then blend with an immersion blender until smooth. Add the butter and blend again until the ganache is glossy and fully emulsified. Pour the ganache over the chilled brownies and tilt the pan gently to spread it evenly. Refrigerate for at least 2 hours, or until the ganache is fully set.

Before serving, sprinkle the top with flaky sea salt. Cut into squares using a sharp knife, wiping the blade clean between cuts for neat edges.