Toast sesame seeds in a skillet over low heat until fragrant. Place in a blender or food processor and blend 5 minutes, occasionally scraping sides. Add oil and purée 5 minutes, or until mixture resembles soft peanut butter. Add 1 cup of water. Process until paste dissolves. Add remaining water, sugar, water chestnut powder and cornstarch. Process until blended. Pour batter into a large bowl.
Fill the bottom of a steamer with water and bring to a boil. Place steamer basket on top. Lightly oil a heavy-duty cake or loaf pan that will fit into the steamer basket.
Pour batter in a thin layer into the pan. Place pan in steamer and steam 4 minutes or until mixture is set. Remove and cool.
Loosen sides with a spatula, then roll up lengthwise. Place seam-side down on a lightly oiled plate or pan. Repeat steaming process with remaining batter, stirring batter and oiling the pan each time.
Slice rolls into 3-inch pieces and serve at room temperature. Rolls may be refrigerated, but steam to soften before serving.