Preheat the oven to 350°F (175°C) and line an 8-inch square pan with parchment. Melt the butter and chocolate together in a heatproof bowl over simmering water, stirring until smooth. Set aside to cool slightly.

In a large bowl, whisk the eggs and both sugars vigorously for 2 minutes, until pale and thick.

Stir the melted chocolate mixture and vanilla into the egg mixture until smooth.

Sift the flour, cocoa powder, and salt into the bowl. Add chocolate chips if using. Fold just until no dry patches remain.

Scrape the batter into the pan and smooth the top. Bake for 25 to 28 minutes, until a toothpick comes out with a few moist crumbs.

Cool completely in the pan, then lift out and slice into 16 squares.