Preheat your oven to 350°F (175°C) and prepare a 9x9 inch baking pan.
Melt the dark chocolate and butter together in a heatproof bowl over simmering water or in the microwave in short bursts.
Remove from heat and stir in both sugars until fully combined.
Mix in the eggs, one at a time, stirring well after each addition. Add vanilla extract.
Sift together flour and salt in a separate bowl, then fold into the chocolate mixture until just combined.
Gently fold in the chocolate chips.
Pour the batter into the prepared baking pan and smooth the top.
Bake in the preheated oven for 20-25 minutes, or until a toothpick inserted comes out with a few moist crumbs.
Remove from the oven and allow to cool in the pan for about 10 minutes, then transfer to a wire rack to cool completely.