Heat the oven to 180C/160C fan/gas 4. Butter a 4cm-deep, 20 x 20cm square cake tin and line with baking parchment. Melt 200g of the chocolate together with the butter in a large heatproof bowl set over a pan of just simmering water (ensure the bottom of the bowl doesn’t touch the water), stirring occasionally. Remove from the heat and leave to cool for 10 mins.
Meanwhile, whisk the sugar, eggs and a pinch of salt together in a large bowl until pale and foamy – it should look like a frothy milkshake. This will take 2-3 mins with an electric whisk, or about 5-8 mins with a balloon whisk. Pour in the butter and chocolate mixture along with the almonds, then fold everything together until there are no streaks of chocolate. Tip the mixture into the tin, scatter over the rest of the chocolate, and bake for 25 mins, until cooked with a slight wobble in the middle.
Leave to cool for 10 mins, then scoop into bowls and serve warm for a pudding. Alternatively, leave to cool completely in the tin, then cut into squares and store in an airtight tin. Will keep for up to five days.