Cut butter into small cubes and break up the chocolate into small pieces. Put together in a large bowl and melt in the microwave or over a hot water bath.
Once melted, fold in the sugar. Mix to combine.
Sieve flour into a separate large mixing bowl.
Pour in the melted chocolate mixture. Mix well.
Add in the eggs. Then, add the vanilla extract and salt. Mix to combine.
Pour mixture into the lined cake tin. Bake in the oven for 35 to 40 minutes.
Allow to rest for 10 minutes before dusting with icing sugar on top (if using) for serving.
Notes
Choose the correct size for your baking tin. A larger tin would mean your batter would be spread more thinly, and hence requiring less time to cook through. Whereas a smaller tin means the batter would be packed into a taller height and would require a longer time to cook through.
Use a spatula to spread the batter evenly across your baking tin. Spreading the batter evenly would help to ensure that the brownies would be baked evenly and rose to an even height.
Do not overbake the brownies. To test whether the brownies are ready, insert a toothpick and it should come out still sticky with a few moist crumbs. If you overbake them, you'd be missing out on the gooey texture of the brownies.
Allow the brownies to cool down for at least 10 minutes before removing them from the tin and cutting them. If you touch them while they're still super hot, you might find your brownies breaking apart into crumbly bits.