Preheat your oven to 325°F (160°C).
Melt the butter and chocolate together in a saucepan over low heat.
Remove from heat and whisk in granulated and brown sugars.
Add the eggs one at a time, whisking between each addition.
Stir in the vanilla extract and mix until smooth.
Sift the flour and salt together, then fold them into the chocolate mixture.
Pour the batter into a greased baking dish.
Bake for 30-35 minutes or until the edges are set but the center appears moist and slightly wobbly.
Let it cool for a few minutes before serving.
