Preheat the oven to 320°F (160°C) convection. Line a 9-by-9-inch/24-by-24 cm square baking pan with parchment paper and lightly grease it with oil spray.

In a medium heatproof bowl set over a saucepan of gently simmering water, melt the chocolate and butter, stirring until smooth and fully combined. (Alternatively, melt them in a microwave in 30-second intervals, stirring between each.)

Remove the bowl from the heat. Add the sugar and eggs to the chocolate mixture and whisk until smooth and glossy. Stir in the flour and salt until the batter is evenly mixed and no streaks remain. Pour the batter into the prepared pan and smooth the top.

Bake for 12 to 15 minutes, until the edges are set but the center remains slightly jiggly. Transfer the pan to a wire rack and let the brownies cool completely, about 1 hour.

Combine the sugar and glucose syrup in a medium saucepan over medium heat. Cook without stirring, swirling the pan occasionally, until the mixture turns a light golden color.

In a separate microwave-safe bowl or measuring cup, warm the heavy cream and sweetened condensed milk until hot but not boiling. Carefully and gradually pour the warm cream mixture into the caramel, stirring constantly. Be cautious—it will bubble vigorously.

Return the caramel to the heat and cook for 2 to 3 minutes over medium, stirring frequently, until slightly thickened. Remove from the heat and gradually whisk in the butter and salt. Blend the caramel with an immersion blender until silky and smooth. Let cool slightly at room temperature.

Pour the caramel over the cooled brownie base and spread it evenly. Refrigerate for at least 2 hours, or until the caramel is fully set.

Sprinkle with flaky sea salt, cut into squares, and serve chilled or at room temperature.