1. In a medium saucepan fitted with a steamer basket, add enough water to come up just below the basket and bring to a simmer over medium heat. Place the sweet potato and yam in the basket and steam, covered, until tender when pierced with a knife, 30 minutes.
2. Transfer the potatoes to a cutting board until cool enough to handle, then peel and press through a ricer.
3. Heat a medium saucepan over medium heat and add the riced potato and yam. Cook until dried slightly and the mixture has formed a single mass, 5 to 6 minutes. Stir in the remaining ingredients to incorporate.
4. Line six 4-ounce ramekins with plastic wrap and press ¼ cup of the mixture into each, smoothing the tops flat. Let cool completely.
5. Use the plastic wrap to remove each cake from the ramekin and cut into wedges, then serve.