Rinse the rice under cold running water until the water runs mostly clear. Transfer to a large bowl, add the cinnamon stick and warm water, cover, and refrigerate for at least 4 hours — ideally overnight. This hydration step softens the grains, extracts cinnamon oils, and ensures the final horchata blends into a naturally creamy base instead of tasting thin or chalky.

Remove most of the cinnamon stick, leaving a few softened fragments with the rice mixture. Add the very finely grated dark chocolate directly into the soaking liquid. Stir briefly to distribute. Chocolate must be almost powder-fine so it melts seamlessly into the blend rather than forming gritty particles.

Blend the mixture in two batches to maintain proper blade movement and prevent uneven texture. Process each batch on high speed for about 4 minutes until the liquid looks pale, milky, and fully emulsified. Pause halfway to scrape down the sides if needed. Under-blending leaves starch particles intact, which will later feel sandy on the palate.

Strain the blended liquid through a fine-mesh sieve or layered cheesecloth into a large pitcher. Press firmly on the solids to extract as much liquid as possible without forcing coarse residue through. This step defines the drink’s silkiness — proper straining transforms the mixture from rustic rice milk into refined horchata.

Stir in the evaporated milk, sweetened condensed milk, vanilla extract, and a pinch of fine sea salt until fully integrated. Add about 2 cups of cold water and mix well. Taste and adjust deliberately — add sugar only if necessary and more water in small increments if the drink feels too dense. The final consistency should be creamy yet pourable.

Refrigerate for 1–2 hours until fully cold. Stir gently before serving to redistribute any natural settling. Serve over ice and finish with ground cinnamon or a cinnamon stick garnish.