Preheat oven to 350°F (175°C). Grease and line an 8×8-inch baking pan with parchment paper.

In a heatproof bowl, melt the butter and chopped dark chocolate together until smooth. Let it cool slightly.

Whisk in the granulated sugar, brown sugar, eggs, and vanilla extract until the mixture is glossy.

Sift in the flour, cocoa powder, salt, and espresso powder. Gently fold together just until no dry streaks remain.

Fold in chocolate chips or extra chopped chocolate if using.

Pour the batter into the prepared pan and smooth the top.

Bake for 25–30 minutes, or until the center is just set but still slightly gooey.

Let cool completely in the pan before slicing into squares.

Enjoy as-is or topped with a scoop of vanilla ice cream.