Preheat oven to 350°F (175°C) or 155°C fan forced.
In a large mixing bowl of a stand mixer, use the paddle attachment to mix together eggs, granulated sugar, light brown sugar, and sea salt for 2 minutes until thick.
In a measuring cup, melt butter in microwave for 30 seconds, 100% power.
Add dark chocolate and stir. Melt for a further 30 seconds, 100% power in the microwave. Stir until butter and chocolate are completely melted and combined.
Pour melted chocolate mixture into the egg and sugar mixture. Add the vanilla essence and mix to combine.
With mixer still running on low (speed 2), sift in the cocoa powder and plain flour. Stir until just combined.
Lightly butter a 9x9-inch pan and line it with baking paper, smoothing the surface so that it sticks to the pan.
Pour the batter into the prepared pan and smooth to distribute evenly.
Bake 40 minutes.
Let brownies cool completely in pan until they reach room temperature, or place pan in the refrigerator for 30 minutes.
Slice into 24 pieces and serve.