Preheat the oven to 190°C/170°C fan/gas 5. Line a 23cm square baking tin with baking parchment.
Put the butter and chocolate into a heatproof bowl over a pan of barely simmering water and leave until they start to melt. Stir regularly, taking care not to burn the chocolate. Once completely melted, remove from the heat and leave to cool a little.
In a large bowl, using an electric whisk on high power, beat the eggs and sugar together until pale and almost doubled in volume. Add the cooled chocolate and butter mix and gently combine, using a figure-of-eight motion to fold the 2 mixtures into one another.
Sift the flour and cocoa powder together and then fold into the chocolate and egg mixture. Again, fold gently using a figure-of-eight motion until all is combined. It will appear dusty at first, but be patient and it will come together. Take care not to overdo the mixing: as soon as you cannot see any dusty flour mix, you are there.
Now add your extra ingredients and gently fold in, reserving a few to scatter over the top if you like. Transfer the mixture to the lined baking tin, levelling it out and pressing any reserved ingredients into the top of the mixture. Bake for around 35 minutes. The top should be just firm, but the middle should be slightly undercooked and gooey: it will continue to cook in the tin once removed from the oven. Leave the tin on a wire rack to cool before cutting into squares.