Preheat the oven to 180C. Grease & line a 9-inch brownie tin with baking parchment, then set aside.
Place the butter & 400g of the chocolate into a heatproof bowl & melt together in the microwave, being sure to take the bowl out & stir the contents every 20-seconds. Be careful not to burn the chocolate. Set aside.
In a large bowl, whisk together the eggs with the caster sugar until just combined. Add the melted chocolate into the bowl & whisk until everything comes together, but be careful not to over-mix. Add the remaining 100g of chopped chocolate into the bowl, along with the pinch of salt, and use a wooden spoon to gently fold them through the mix.
Pour the batter into the prepared tin & bake in the pre-heated oven for 45-minutes, or until flakey on top and still a little wobbly in the middle. The brownies need to cool in the tin for at least 1-hour, and after being removed should be kept in the fridge before slicing. Enjoy at room temperature with some ice cream dolloped on top.