Half fill a cocktail shaker or blender with crushed ice (cubes will do if crushed ice isn't easily available). Next, pour in 4 measures of milk. For a really indulgent version, you can substitute 1, 2, 3 or all 4 measures of milk for the equivalent amount of single or even double cream. If I'm using double, I usually mix it half-&-half with full fat milk. Alternatively, for a "skinny" version, you could use only 4 measures of skimmed or low-fat milk.