Start by preheating your oven to 175°C (350°F). You want it nice and warm to bake those brownies evenly. Grease a square baking pan (approximately 9-inch) and line it with parchment paper for easy removal later.
In a heatproof bowl, combine the chopped dark chocolate and cubed unsalted butter. Place this over a pot of simmering water, making a double boiler. Stir until both melt into a luscious mixture. Remove from heat and let it cool slightly.
Once the chocolate-butter blend has cooled, whisk in the superfine sugar. This should take just a few minutes. You want the mixture to become shiny and slightly thick.
Add your three eggs and the separated egg yolk. Make sure everything is well combined. This not only adds richness but also helps to aerate the mixture.
Sift together all-purpose flour, sea salt, and espresso powder. Gently fold this mixture into the wet ingredients. Do not overmix; a few flour streaks are perfectly fine. This careful folding helps maintain that gooeyness.
Pour the brownie batter into the prepared pan. Smooth the top with a spatula for evenness. Bake in the preheated oven for about 20-25 minutes. You want the edges set but the middle will still jiggle slightly.
Remove from the oven and let the brownies cool in the pan for at least 10-15 minutes. Once cooled, lift them out using the parchment paper. Cut into squares and indulge.