Heat the oven to 200C/180C fan/gas 6. Butter a 20cm square tin and line with baking parchment. Melt the butter in a large saucepan on low heat. While that melts, put the chocolate into a large bowl near the hob.

Once the butter is melted, turn the heat up slightly. Cook for 3-4 mins, stirring all the while, until the butter is foaming, golden and smells a little like digestive biscuits. Once it is at this stage, remove from the heat and pour straight onto the chopped chocolate. Leave for a few seconds, then stir to melt the chocolate. It should be dark and shiny. Leave to cool.

Whisk the sugar and eggs together in a bowl using an electric whisk until the mixture has doubled in volume and a ribbon holds for 3 seconds when you lift out the beaters. Steadily pour the cooled chocolate mixture into the egg mixture, and fold gently until combined.

Sift in the flour, cocoa powder, espresso powder and a pinch of salt. Mix to combine, then fold in most of the milk chocolate, pretzels and nuts, reserving some for the top. Scrape the batter into the prepared tin and scatter with the reserved chocolate, pretzels and nuts. Bake on the middle shelf of the oven for 20-25 mins until just set with a shiny top and gentle wobble in the middle. Leave to cool completely in the tin, then slice and serve.