Add the 120g of light brown sugar, 240ml of water, 80g of tamarind paste, and the sliced fresh chilies to a small saucepan. Place over medium heat. Stir continuously as the mixture heats, working the tamarind paste into the surrounding water and sugar — tamarind concentrate requires this active stirring to fully loosen and distribute rather than sitting as a dense mass at the bottom of the pan. Bring to a gentle simmer and cook for 5–8 minutes, continuing to stir occasionally. Do not allow the mixture to reach an aggressive boil — vigorous boiling drives off the tamarind's most volatile aromatic compounds and can develop a slightly scorched, flat note in a sugar-based syrup at the high temperature end. The goal is a steady, gentle simmer that deepens the tamarind's character and fully dissolves the brown sugar into a unified, glossy syrup. Tamarind paste or concentrate — made from the dried pulp of the tamarind pod, a leguminous tree fruit used throughout Mexico, South Asia, and Southeast Asia — has a specifically complex flavour profile that makes it unlike any other souring agent in this collection. Its primary acids (tartaric acid and malic acid) produce a sharp, deeply sour character; its significant natural sugar content balances this with sweetness; and its unique aromatic compounds contribute the specifically earthy, slightly dried-fruit, almost smoky depth that makes tamarind immediately identifiable and irreplaceable. The brown sugar in this syrup specifically amplifies the tamarind's own natural caramel-adjacent warmth in a way that white sugar's neutral sweetness would not.

Remove the saucepan from the heat. Add the zest of 1 lime immediately. Cover and allow to steep for 10–15 minutes. The same principle applied across this collection: lime zest's aromatic volatile compounds (primarily limonene and various terpene esters) evaporate within seconds at cooking temperature but infuse fully into a warm, covered liquid at off-heat steeping temperature. The 10–15 minutes of off-heat steeping with the chilies still present also extracts additional chili character into the deepening syrup.

Strain the syrup through a fine-mesh sieve into a clean jug, pressing lightly on the tamarind and chili solids to extract maximum flavour without pressing so hard that the chili seeds' bitter compounds over-extract. Discard all solids. Allow the strained syrup to cool for 5–8 minutes until it is warm but no longer hot — approximately 50–60°C. Stir in 80ml of fresh lime juice. Taste immediately. The syrup at this stage should taste specifically assertive — intensely sweet-sour from the tamarind, warm and building in heat from the chili, earthy and complex, and brightened by the lime. The flavour should feel almost too bold in isolation: this is the correct calibration for a concentrate that will be poured at 60ml per glass and then diluted by 120–150ml of club soda and crushed ice. A syrup that tastes pleasantly balanced before dilution will taste flat and mild in the glass. Adjust: if additional lime brightness is needed, add the remaining 20ml of lime juice; if the sweetness is insufficient for the tamarind's assertiveness, the brown sugar quantity can be slightly increased in the next batch. Allow to cool completely to room temperature, then refrigerate until thoroughly cold.

Combine the 1 tsp of chili powder, 1 tsp of fine sea salt, and optional ½ tsp of brown sugar on a small plate — mixing with a finger to distribute evenly. The optional brown sugar is specifically the addition that shifts the rim from the standard Mexican chili-salt (Tajín-adjacent) character into something slightly more balanced — the sweetness specifically tempering the rim's salinity and making each rimmed sip specifically more pleasant rather than simply salty and spiced. Take each glass and rub the rim partially — approximately half the circumference — with a used lime half or lime wedge, applying gentle pressure so the lime juice coats the glass rim consistently. Dip the rubbed section into the chili-salt mixture. The partial rim is the specific technique decision that separates a thoughtful rimmed drink from an aggressively over-rimmed one. Rimming the entire circumference of a glass means every single sip carries the chili-salt character regardless of preference — the rim dominates the experience. Rimming half the circumference allows the drinker to rotate the glass, choosing each sip between the clean sparkling drink from the unrimmed side and the specifically savoury, spiced hit from the chili-salt edge. This choice-availability makes the drink more interesting and more personal to each person's preference in the moment.

Fill each rimmed glass generously with crushed ice — crushed ice rather than cubed ice is the specific choice for this preparation. Crushed ice melts faster and chills the drink more immediately on contact, diluting the assertive syrup progressively as the drink is consumed rather than concentrating it heavily at first and then diluting it unevenly. The progressive dilution specifically works in this mocktail's favour — the first sip is the most intensely tamarind and lime; subsequent sips become lighter and more sparkling as the ice continues melting through the glass. Pour 60ml of the chilled tamarind chili syrup over the crushed ice. If additional lime brightness is desired — recommended for those who enjoy vivid citrus — add 15–20ml of fresh lime juice directly into the glass. Top with 120–150ml of chilled club soda or sparkling water, poured gently. Stir once or twice with a bar spoon. Add a lime slice inside the glass against the ice and a lime wedge on the rim for squeezing. Serve immediately.