Preheat the oven to 180C/160C Fan. Grease and line a 20cm (8-inch) square tin with non-stick baking paper and set aside.
Put the diced butter and chopped dark chocolate into a heat-proof bowl. Set over a pan of barely simmering water, ensuring the bottom of the bowl doesn’t come into contact with the water (or the chocolate will overheat) and allow it to slowly melt, stirring often until smooth. Alternatively, pop into a microwave-safe bowl and melt in 20 second bursts, stirring between each. This is the quickest method but you have to watch closely and ensure it doesn’t overheat or the chocolate will burn and you will have to start again. Once melted, set aside to cool slightly.
Stir in the sugar until dissolved, then add the eggs and give it a good mix.
Sift in the flour and a pinch of salt. Add the coffee and vanilla extract, if using, and gently fold together until just combined to form a smooth batter. You want to avoid too much stirring once you've added the flour as it can make the brownies tough.
Fold through half of the white chocolate chunks until evenly distributed throughout the batter. Again, be mindful of not stirring too much.
Pour into the prepared tin. Scatter over the other half of the chocolate chunks and use a butter knife or spoon to push the chunks into the batter. Smooth over the surface. Now here’s the tricky bit… Bake in the oven for 25-35 minutes. The exact length of time will depend a) on your oven and b) how you like your brownies. For a fudgy, gooey brownie, you will need a shorter time. Remember, it will continue to bake after you remove it from the oven. You are looking for a just ‘set’ crinkly top with a slight wobble in the centre (the more gooey you like your brownies, the more wobble you want). It can be a bit of trial and error on the first attempt - but it is hard to think of a more delicious experiment!
Remove from the oven and leave to cool in the tin completely. If they are very gooey, once cooled, cover and chill in the fridge to firm up.
Remove from the tin and cut into 16 squares or 24 bite-sized pieces. Enjoy!