Wash and soak the glutinous rice and white rice in water for at least 30 minutes. Drain well.
Roast the black sesame seeds in a dry pan over medium-low heat until fragrant, about 5-7 minutes. Be careful not to burn them. Let them cool.
Combine the roasted black sesame seeds, soaked glutinous rice, soaked white rice, and 500ml of water in a blender. Blend until very smooth.
Strain the blended mixture through a fine-mesh sieve into a pot, pressing down on the solids to extract as much liquid as possible. Discard the solids.
Add the remaining 500ml of water, rock sugar, and pandan leaf (if using) to the pot with the strained sesame liquid.
Heat over medium heat, stirring continuously to prevent sticking, until the paste thickens to your desired consistency. This usually takes 10-15 minutes.
Remove the pandan leaf and serve the black sesame paste warm.
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