In a small saucepan, whisk together the cocoa powder, sugar, and water. Place over medium heat and stir until smooth and fully dissolved. Add the chopped dark chocolate and the pinch of salt. Stir until melted and glossy, then remove from the heat and stir in the vanilla. Let the sauce cool slightly. The sauce should be smooth, shiny, and pourable — not grainy and not thick like fudge. If it is too thick, it will sit in clumps at the bottom of the drink instead of blending properly. Building the sauce from cocoa powder dissolved in warm water first, then finishing with chopped dark chocolate melted into that base, produces a considerably smoother, more pourable result than melting chocolate alone — the cocoa-and-sugar base carries enough liquid to keep the whole sauce fluid rather than seizing into something thick.

Brew 120ml espresso total, equal to 4 standard 30ml shots. While still hot, stir 40ml of the chocolate sauce into the espresso until fully combined. The hot espresso should dissolve and loosen the chocolate sauce into a unified dark mocha base. If the sauce is added only to cold milk later, it blends less cleanly. Combine the chocolate and espresso while the coffee is still hot. That is what gives the drink a smooth base rather than separate chocolate streaks floating through it. Hot espresso has considerably more capacity to loosen and fully dissolve the sauce than cold milk would — stirring the chocolate in at this stage, while the temperature and liquid are working in the sauce's favour, is what produces a genuinely unified, streak-free mocha base.

Drizzle a little chocolate sauce around the inside walls of 2 glasses in a loose pattern so it stays visible from the outside. Fill each glass generously with ice. The chocolate drizzle should cling visibly to the glass without pooling into a thick heavy layer at the bottom. It is there for the classic visual and a little extra chocolate impact, not to overload the drink. Applying this drizzle to the empty, dry glass before any liquid is added is what allows it to cling directly to the glass walls — added later, once the glass already holds ice or milk, that same sauce would slide down and disappear rather than remaining a visible accent.

Divide the mocha-espresso mixture evenly between the 2 glasses. Pour about 120ml cold milk into each glass, holding back roughly 40ml milk total for the top foam. Stir each glass briefly so the milk and mocha base combine. The drink should become a deep chocolate-brown colour, clearly darker than a regular iced latte. If it looks too pale, the coffee-chocolate base is too weak or there is too much milk. Holding back a small, specific amount of milk at this stage rather than using it all in the main pour is what makes the separate, lightly frothed foam topping possible in the next step — that reserved portion is deliberately set aside for a different job than the rest of the milk.

Combine the reserved cold milk with 1 tsp chocolate sauce. Froth lightly using a handheld frother, small milk frother, or by shaking briefly in a sealed jar. You only want a thin, light foam, not a thick cappuccino cap. The foam should be soft, lightly aerated, and chocolate-tinted, with fine bubbles and a thin spoonable texture. If it is thick and dry, it is over-frothed for an iced mocha. This restrained, brief frothing is a genuinely different target than the thick, dry foam a cappuccino calls for — stopping well short of that fuller aeration is what keeps this foam reading as a light finishing touch rather than a substantial topping layer.

Spoon or pour the thin chocolate milk foam over the top of each drink. Dust lightly with cocoa powder and serve immediately. The final drink should show three clear visual cues: deep dark chocolate-coffee colour, visible chocolate swirls on the inside of the glass, and a thin dark chocolate milk foam with a light cocoa dusting on top. Serving without delay is what preserves all three of those visual cues together — the foam's light texture softens and the chocolate swirls on the glass begin to blur into the drink the longer it sits.