Heat oven to 180˚C, gas mark 4. Grease and line a 30cm x 20cm baking tin with baking parchment.
Melt the chocolate and butter in a large heatproof bowl set over a pan of barely simmering water. Remove from the heat and set aside. Whisk the eggs, sugar and salt in another bowl, before beating into the chocolate mixture. Sift over the cocoa and flour, folding through with all but a handful of the Christmas pudding, until just combined.
Pour the brownie mix into the prepared tin, spread out and scatter the rest of the Christmas pudding on top, pressing it in gently. Bake for 35-40 minutes, or until a crust has formed on top. Cool completely in the tin before slicing and serving.