Combine the 65g of white sugar and 240ml of water in a small saucepan. Stir over medium heat until the sugar is completely dissolved and the liquid is clear. Remove from the heat immediately — no reduction, the same standard simple syrup approach from the classic still lemonade. The 65g quantity is specifically calibrated for the sparkling version: carbonation's effect on sweetness perception is measurable and consistent. Carbon dioxide dissolved in water forms carbonic acid, which specifically interacts with the sweet taste receptors to enhance perceived sweetness from dissolved sugars. The effect is approximately equivalent to a 15–20% increase in perceived sweetness at typical sparkling water carbonation levels — meaning 65g in sparkling lemonade produces approximately the same perceived sweetness as 80g in still lemonade. Allow to cool completely.

Juice the lemons until you have 220–250ml of fresh juice — slightly less than the classic still version for the same reason the sugar is less: carbonation enhances the perception of acidity as well as sweetness. At the same lemon juice quantity, a sparkling lemonade tastes more specifically tart than the still version. The 220–250ml quantity at this syrup level produces the correct balance in the sparkling format. In a pitcher, combine the lemon juice, approximately two-thirds of the cooled simple syrup — approximately 42ml — and the ⅛ tsp of fine sea salt. Stir well. This combined mixture is the lemon base — more concentrated, more specifically tart, and more intensely flavoured than the intended finished drink. It should taste sharper and more assertive than a comfortable-to-drink lemonade. This concentration is correct: it will be significantly diluted and its perceived intensity amplified when the sparkling water adds both volume and carbonation. Transfer the pitcher with the lemon base to the refrigerator and chill for 1–2 hours until completely cold. The base must be completely cold before the sparkling water is added — adding sparkling water to a warm base immediately begins degassing the dissolved carbon dioxide as the temperature differential causes rapid bubble formation.

Remove the chilled lemon base from the refrigerator immediately before serving. Pour the 1 litre of ice-cold sparkling water gently down the side of the pitcher rather than pouring directly into the centre — the side pour disperses the sparkling water's energy against the pitcher's wall rather than directing it into the lemon base, releasing significantly less carbonation than a direct pour. Stir once or twice only, very gently, with a single slow movement rather than a vigorous stir. The goal of the stir is to combine the lemon base's concentrated flavour evenly through the sparkling water; more than two gentle stirs depletes the carbonation meaningfully. Taste immediately. If the acidity feels too aggressive, add a small additional amount of the remaining simple syrup — pouring it gently along the side and stirring once. If the concentration is too intense, add a splash more sparkling water up to 1.2 litres total. Fill glasses with ice immediately — the shorter the time between adding the sparkling water and serving, the more carbonation remains in the glass. Pour the sparkling lemonade over the ice from close range — pouring from a height releases carbonation during the fall. Garnish with a lemon slice. Serve immediately while cold and fully fizzy.