Add the brown sugar, water, salt, and cinnamon if using to a small saucepan. Warm over medium heat, stirring until the sugar fully dissolves. Simmer briefly for 30–60 seconds, then remove from the heat and let it cool slightly. The syrup should be fully dissolved, smooth, and pourable — not grainy and not thick like caramel. Brown sugar that is not fully dissolved will not distribute evenly through the drink. Brown sugar's molasses content makes it slightly more resistant to dissolving cleanly than white sugar, which is exactly why the full warming and brief simmer matter here — rushing this step risks leaving fine undissolved grains that never fully integrate once the syrup meets cold liquid later on.

Brew 120ml espresso total, equal to 4 standard 30ml shots. Use the espresso while still fresh and hot. If it sits too long, the flavour flattens and the shaken foam becomes less lively. The espresso should taste bold and clean, not burnt. This drink needs a strong coffee base because oat milk and ice soften the intensity quickly. Both the shaking stage and the oat milk topping that follows work to dilute and cool the espresso considerably — starting with a genuinely bold, fresh shot is what ensures real coffee character survives all the way through to the finished drink.

Divide a small amount of the brown sugar syrup between the 2 glasses, about 5ml per glass. This creates the visible deeper syrup layer at the base. Fill both glasses generously with fresh ice. The syrup should sit at the bottom clearly, while the glasses are packed enough with ice to support the gradient and keep the drink cold without becoming watery immediately. This small syrup reservoir at the very base is a deliberate visual anchor — its dense, slow-dissolving pool is what gives the finished drink its darkest, most concentrated point at the bottom of the glass.

In a shaker, combine the hot espresso with 30ml of the remaining brown sugar syrup and a generous handful of ice. Shake hard for 10–15 seconds until the shaker feels very cold and the espresso becomes frothy and aerated. This is the core step. The mixture should lighten slightly in colour and develop a pale caramel-amber foam on top. If you barely shake it, you do not get the classic shaken espresso texture. The espresso must be shaken while still hot with ice. That rapid chilling and aeration is what creates the proper frothy top. Shaking the syrup directly into the espresso at this stage, rather than adding it separately later, means the majority of the drink's sweetness gets fully whipped and integrated into the coffee itself — this is what keeps the finished drink tasting cohesive rather than sweet in one spot and plain elsewhere.

Strain or pour the shaken espresso evenly into the 2 prepared glasses over the fresh ice. Slowly top each glass with 120ml cold oat milk. Do not stir immediately if you want the proper visual gradient. Let the oat milk move through the espresso naturally so the drink shows a pale upper layer, darker lower tones, and a visible syrup concentration at the base. The drink should show a gradient through the glass, with the shaken espresso foam sitting lightly at the top and the oat milk softening the colour into a pale caramel tone. Pouring the oat milk in slowly rather than all at once lets it settle and layer over the denser, already-shaken espresso rather than immediately blending through it — that gentle pour is what preserves the visible gradient through the glass.

Serve immediately. Stir before drinking if desired, but the classical presentation is to serve it with the gradient intact first. Presenting the drink in its naturally layered state before stirring is what showcases the visual effect the whole build was designed to create — that layered moment fades quickly once the drink sits, so serving right away matters for both freshness and presentation.