Need to make a celebration caramel icing at short notice? Butter two 20cm springform tins and line the bases with baking parchment. Beat the butter and both sugars in a bowl with an electric whisk for a few mins until lighter in colour and fluffy.
Add the vanilla and the eggs, one at a time, adding a spoonful of flour and beating in between each egg. Add the remaining flour and milk. Meanwhile, for the icing, put the butter and icing sugar in a bowl and whisk for a few mins until light and airy. Whisk in the caramel briefly, adding 1 tbsp of boiling water to loosen, if needed. Set aside until the sponges are completely cool before assembling, or the icing will melt. Use half the icing to sandwich the cakes together, then spread the remainder over the top, smoothing it out with a knife or the back of a spoon.