Put the butter in a pan – ideally one with a light-coloured base, so you can see when the butter darkens – set it over a medium heat and, once the butter has melted, leave it to simmer, swirling the pan around from time to time so it cooks evenly.
You want to toast the solids in the butter, so cook until they turn golden brown and start smelling nutty (the bubbles will start shrinking at this point, too). Take off the heat and pour into a large bowl.
Heat the oven to 180C (160C fan)/350F/gas 4 and line a 20cm square baking tin with greaseproof paper. In a medium bowl, whisk the sugar and eggs until fluffy, then whisk these into the melted butter.
In a separate bowl, whisk the flour, baking powder, salt and ground cardamom, fold this through the butter mix, then fold in the chopped dates and chocolate.
Pour the batter into the prepared tin, smooth the top and bake for 30-35 minutes, until risen and golden but still with a gentle wobble in the middle. Remove and leave to cool a little before eating.