Preheat the oven to 350°F. Line an 8x8 pan with parchment paper.
Whisk melted butter with sugar until the sugar is mostly dissolved and the mixture is smooth.
Whisk in eggs, vanilla extract and espresso powder for around 1 minute. If you're using eggs cold from the fridge, it'll start off a bit gloopy, so make sure to whisk it until it's smooth.
Whisk in melted chocolate. Make sure the chocolate isn't hot or else it will cook the eggs.
Use a rubber spatula to fold in the flour, cocoa powder and salt, until just combined. It should be thick and mostly smooth.
Fold in the chopped chocolate until just combined.
Pour the batter into the prepared pan and smooth out.
Bake for 25-30 minutes, until the edges are set. The middle will have a thin crust, but will feel very soft if you poke it gently.
Let cool at room temperature before slicing into squares.