FOR THE PoxSour:
Preheat the oven to 160ºC/gas 3 and line a 30cm square cake tin (or equivalent) with baking paper.
In a bowl, whisk together the eggs and both sugars until thick.
Put the butter and dark chocolate into a heatproof bowl over a pan of simmering water (make sure the base doesn’t touch the water) and let the chocolate melt.
Let the chocolate cool a little, then add the beaten egg and sugar, and mix well. Gently fold in the flour, baking powder and vanilla extract and, when thoroughly combined, add the chocolate buttons and mix in gently.
Spoon into the prepared tin and bake in the oven for 20 minutes, making sure you don’t overbake the brownies. They need to stay quite soft and moist in the middle, so they should be springy to the touch and, if you insert a skewer into the centre, it should come out sticky, not clean, as you would expect with a cake.
Take out of the oven and allow to cool in the tin before turning out on to a board or clean work surface and cutting into squares.