STIR powdered sugar with lemon juice in base of shaker until sugar dissolves. Add gin, SHAKE with ice and strain into ice-filled glass. TOP with champagne and lightly stir.
Judge Jr’s original 1927 recipe calls for "2 jiggers Gordon water; 1 part lemon juice; a spoonful of powdered sugar; cracked ice. Fill up the rest of a tall glass with champagne!" I’ve loosely interpreted this recipe to the one above (with the 3 spoons of powdered sugar being flat 5ml measuring spoons). The result is dangerously quaffable gin spiked champagne with a zesty lemon bite. The powered sugar adds an attractive sherbet note, but if you prefer to use sugar syrup then substitute with ½oz/15ml (2:1).
Adapted from a recipe in the 1927 book Here's How by Judge Jr..