In a medium bowl, whisk the yam, toasted coconut, sugar, egg whites, salt, and lemon zest together.
In a food processor, pulse the almonds to an uneven meal ranging in texture from very fine (mostly) to finely chopped. Stir the almonds into the batter. The dough will be very soft and sticky. Chill for at least 1 hour and up to 2 days to allow the coconut to absorb moisture from the yams.
Preheat the oven to 325°F. Position racks in the upper and lower thirds of the oven. Scoop level tablespoons of the soft dough and roll into balls about 1 1/4 inches. Roll the balls in sugar (or, if using turbinado sugar roll only the top half of each ball in sugar to avoid oversweetening). Place 1 1/2 inches apart on the lined or greased cookie sheets.
Bake for 18 to 24 minutes, until the cookies are slightly crusty on the surface (though still very tender and moist inside) and deep golden brown on the bottom. Rotate the pans from top to bottom and from front to back halfway through the baking time to ensure even baking. For lined pans, set the pans or just the liners on racks to cool; for unlined pans, use a metal spatula to transfer the cookies to racks. Cool the cookies completely before storing. May be kept in an airtight container for up to 4 or 5 days.
Cardamom Pannelets: Omit the lemon zest. Add 1 1/2 teaspoons ground cardamom, 2 teaspoons grated orange zest, and 2 teaspoons orange flower water to the yam mixture.
Masala Pannelets: A little Spanish/Asian fusion cookie and very good. Omit the lemon zest. Add 2 teaspoons garam masala to the yam mixture. Roll in plain sugar or cinnamon sugar.