In a non-stick frying pan, fry the sesame seeds on medium low heat for about 10 minutes, until fragrant and pan begins to smoke. Remove and cool completely.
Drain the rice and blend with enough water until mixture is smooth. Remove and clean blender.
Repeat this process with the black sesame seeds until mixture is a grayish paste and free from lumps.
In a large pot, add the grinded ingredients, pandan leaves and sugar.
Stir to mix and bring to a boil on high heat. Lower heat to medium, stirring the mixture until it thickens slightly.
Add in sugar and butter to mixture and more water if mixture get to thick.
Simmer for another 15 minutes until mixture is smooth, season to taste with sugar if needed.
Remove pandan leaves before serving.