Sift 200 g caster sugar (or icing sugar for finer dissolve) and 60 g unsweetened cocoa powder (dutch-processed for the classic milder, less acidic taste) together into a bowl to remove lumps — cocoa clumps badly and this is the step people skip.
Add 1 tsp ground cinnamon, 1 tsp sunflower or soy lecithin (the emulsifier — helps it disperse instead of clumping on top of milk), and 1 pinch fine salt. The lecithin is doing the real work here — without it you'll get the classic 'cocoa raft' floating on top no matter how hard you stir.
Pulse everything in a spice grinder or food processor for 20-30 seconds so the lecithin (which comes as small granules or a powder) is evenly distributed, not sitting in pockets.
Store in an airtight jar. Keeps for months since it's just dry ingredients.
Use roughly 2-3 tablespoons per 250ml cold milk. Whisk or shake in a bottle — cold milk shows off the emulsifier's job better. Warm.