In a small saucepan over medium-low heat, melt the butter with ¾ cup of the sugar, stirring until glossy and smooth. Bring to a gentle simmer and cook for 1–2 minutes, until syrupy and mostly dissolved. Remove from heat and let cool slightly.

In a large bowl, beat the eggs, egg yolk, and remaining ¾ cup sugar until thick, pale, and creamy, about 2–3 minutes. Beat in the vanilla.

With the mixer on low, slowly stream in the warm butter mixture and mix until fully combined and glossy.

Sift in the cocoa powder, flour, espresso powder (if using), and salt, mixing just until incorporated.

Fold in the melted chocolate, then gently fold in the chopped dark chocolate.

Spread the batter evenly into the prepared pan and smooth the top. Sprinkle with additional chocolate if desired. Bake for 35–40 minutes, until the top is shiny and crackly, the edges are set, and the center still has a slight jiggle.

Remove from the oven, sprinkle with flaky sea salt if using, and cool completely before slicing for the fudgiest texture and clean edges.