Peel and slice the purple sweet potatoes. Steam the purple sweet potatoes until very tender.
Mash the steamed purple sweet potatoes into a smooth paste.
In a large bowl, combine the glutinous rice flour, corn starch, granulated sugar, and salt.
In a separate bowl, whisk together the thick coconut milk and water.
Gradually add the mashed purple sweet potatoes to the dry ingredients, mixing well.
Slowly pour the coconut milk and water mixture into the sweet potato and flour mixture, stirring until a smooth, lump-free batter is formed.
Lightly grease an 8-inch round steaming pan. Pour the batter into the prepared pan.
Steam the cake over high heat for 30 minutes, or until a skewer inserted into the center comes out clean.
Let the cake cool completely before slicing. Garnish with shredded coconut before serving.
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