Heat the oven to 160C fan. Line the baking tin with parchment paper.
Begin by placing the butter in a large heat proof bowl with the chocolate and sea salt, set over a saucepan with barely simmering water. (Don’t let the bowl touch the water). Stir occasionally, as soon as the mixture has melted remove from the heat. Cool for 5-8 minutes.
In the meantime in a free standing mixer beat the eggs and both types of sugar until really light and fluffy. Combine the chocolate mixture with the sourdough starter, rye flour and cocoa, stir until you have a smooth batter. Then pour the fluffy egg mixture over the chocolate mixture. Fold and stir until you have a smooth batter.
Pour the mixture into the prepared tin and add the nuts and chocolate if using.
Place in the centre of the oven and bake for exactly 20 minutes. Remove from the oven and sprinkle with Malden salt while the chocolate chunks are hot. Cool completely.
Cut into squares when you're ready to serve.