Prepare a small saucepan and combine Agar-Agar Powder with Water; let the agar sit for a few minutes to hydrate while you measure the other ingredients.
Heat the mixture over medium, stirring constantly until the agar fully dissolves and the liquid comes to a gentle boil, then reduce the heat and simmer briefly to ensure the powder is completely melted.
Stir in the Sugar and a small pinch of Salt, continuing to cook until the sugar is fully dissolved and the liquid is smooth and clear.
Lower the heat and whisk in the Koshian (Smooth Red Bean Paste) in batches so it blends evenly into the agar syrup; keep stirring until the mixture is homogeneous and slightly glossy.
Optional: pass the mixture through a fine sieve into a heatproof bowl or measuring cup to remove any lumps and ensure an ultra-smooth texture.
Pour the mixture into a lightly oiled or lined mold (a loaf pan or square mold works well), smoothing the surface with a spatula.
Let the yokan cool to room temperature, then refrigerate for at least 2–3 hours (or until fully set) until it is firm to the touch.
To serve, run a thin knife around the edges, invert if using a mold, and slice into neat pieces. Keep refrigerated and enjoy chilled with green tea.
