Butter two 8 in / 20.5 cm sandwich cake tins, and line the bases with greaseproof paper. Cream the butter and sugar together until pale and fluffy. Beat in the egg yolks, one at a time, sprinkling on some of the measured quantity of flour and mixing thoroughly after the addition of each egg yolk to prevent curdling. Mix in the syrup and milk, and then fold in the remaining flour and the salt. Whisk the egg whites to peaks, and then gently fold into the cake batter with a large metal spoon. Divide the mixture between the cake tins and level the surface with the back of a spoon. Bake in a preheated oven at 180°C / 350°F / Mark 4 for 30–35 minutes, until a warmed skewer inserted in the centre of each cake emerges clean. Remove from the oven, turn out and cool on wire racks. The cakes, when cold, can be stored in an airtight container and assembled the next day.
To make the caramel, put the sugar, cream and butter in a saucepan, and heat gently until the sugar has melted. Bring to the boil, stirring continuously, and boil for 7 minutes. Away from the heat, beat the caramel to thicken it. Sandwich the two cakes with some of the caramel and the chopped walnuts. Pour the rest of the caramel over the top of the cake, and decorate with walnut halves.