Adapted from Handle the Heat
Preheat the oven to 175°C and line an 5x5-inch baking pan (or your smallest pan) with parchment.
In a small microwave bowl, combine the two types of chocolate and butter, and unsweetened chocolate and heat in the microwave for 30 second intervals until it is melted.
In a different bowl beat with the eggs and sugar with a mixture for 2 minutes then add the chocolate mixture, flour, and salt and stir together with a spatula until combined.
Pour the batter into pan and bake until a tester comes out clean, about 25-30 minutes and then allow to come to room temperature before cutting.