It takes about 3 hours to make homemade red bean paste, so feel free to use store-bought. It comes in smooth (koshian) and chunky (tsubuan) versions; the smooth is easier to work with and is a nicer match for strawberries. Shiratamako and mochiko are both glutinous rice flours. Shiratamako’s flavor and texture is preferable for mochi, but you can substitute the less expensive and more widely available mochiko in a pinch. Shiratamako has a coarse, pebbly texture, but don’t worry; it will dissolve smoothly into the warm mochi batter.