Melt the butter in a small pan, using a bain-marie, and then leave it to cool down. Coarsely chop the walnuts, with a knife on a chopping board. Beat the sugar and eggs with a whisk, add the cold melted butter, milk, flour and baking powder (sifted together) and the walnuts, stirring constantly until the mixture has a smooth, even texture. Grease a round cake tin (28 cm in diameter) with butter and dust it with flour, pour in the mixture, level the surface and bake in a preheated oven, at 180 °C, for 40 minutes. Before taking the cake out, check to see if it’s cooked: stick a toothpick into the centre – pushing it well in – and if it’s dry when you pull it out, the cake is ready. Let it cool down, remove it from the cake tin and sprinkle with icing sugar.