Preheat oven to 180°C.
Add chocolate (chips) and butter into a mixing bowl, then place over a pot of simmering water (bain marie), ensuring that the water doesn't touch the bottom of the mixing bowl. As the chocolate and butter melt, stir until well-combined. Remove from heat and allow the mixture to cool.
In another mixing bowl, whisk sugar and eggs using an electric mixer until pale and fluffy. This should take around 3-5 minutes. Add in vanilla extract and mix well.
Add chocolate and butter mixture into the egg and sugar mixture and combine using a rubber spatula. Once well-combined, fold in flour and salt gently.
Line a 18-20cm square baking tin (height of 4cm) with greaseproof paper, then pour brownie mixture into tin. Tap baking tin against table to release some air bubbles that are trapped at the top. Insert baking tin into oven and bake for around 20-30 minutes. Insert a skewer into brownie to check if ready - the skewer should have some moist crumbs attached to it.
Note: The baking time will differ depending on your oven temperature, the size and height of your baking tin, as well as where the tin is placed in the oven, so adjust the baking time accordingly.