Preheat the oven to 180C Fan and grease 9 holes of a muffin tin with butter (or use paper cases if you are worried about them sticking).
Put the butter and 140g chopped dark chocolate into a medium 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). Set aside to cool slightly.
Meanwhile, place the eggs and sugars in a large mixing bowl and beat with an electric whisk on medium-high speed for about 1 minute - or until pale, thick and roughly doubled in size.
Add the chocolate-butter mixture and vanilla extract and mix until combined.
Sift in the flour and salt, then fold in until you have a smooth batter. Add the chopped chocolate, if using, and mix until evenly distributed throughout the batter.
Scoop into the prepared muffin tins (I like to use an ice cream scoop) - you should make about 9 brownie bites.
Bake for about 13-15 minutes - or until the tops are puffed up. Remove from the oven and allow to cool in the tin.
Serve with a cup of tea or coffee - or as a pudding with double cream or ice cream and fresh berries.