• Preheat the oven to 325°F. Line an 8x8-inch baking pan with parchment paper, leaving an overhang for easy removal.

Melt the butter and sugar:

• In a small saucepan over medium-low heat, combine the butter and ¾ cup of the sugar. Stir until melted and glossy, then bring to a gentle simmer. Continue cooking 1–2 minutes, until the mixture looks syrupy and the sugar is mostly dissolved (it should reach about 230°F). Remove from heat and let cool slightly.

Whip the eggs:

• In a large mixing bowl, beat together the eggs, egg yolk, and remaining ¾ cup sugar on medium speed until thick, pale, and creamy — about 2–3 minutes. The mixture should form ribbons when lifted. Beat in the vanilla.

Combine:

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

Add the dry ingredients:

• Sift in the cocoa powder, flour, and salt (and espresso powder, if using). Mix just until combined — don’t overmix.

Fold in the chocolate:

• Use a spatula to fold in the melted chocolate.  Then fold in the chocolate chips or chopped chocolate until evenly distributed.

Bake:

• Spread the batter evenly into the prepared pan and smooth the top. Top with more chopped chocolate or chips. Bake for 35–40 minutes, or until the top is shiny and crackly, the edges are set, and the center still has a slight jiggle.

Finish:

• Remove from the oven and immediately sprinkle with flaky sea salt if desired. Cool completely before slicing for clean edges and the ultimate fudgy texture.