Whisk together mashed sweet potato, toasted coconut, sugar, egg whites, salt, and lemon zest in a medium bowl.

Pulse almonds in a food processor until unevenly ground, ranging from very fine to finely chopped.

Stir the processed almonds into the sweet potato mixture, forming a soft and sticky dough.

Chill the dough for at least 1 hour (up to 2 days) to allow coconut to absorb moisture.

Preheat oven to 325°F (160°C) and position racks in the upper and lower thirds.

Scoop level tablespoons of dough and roll into 1 1/4-inch balls.

Roll the balls in sugar (or turbinado sugar on top half only).

Place cookies 1 1/2 inches apart on lined or greased cookie sheets.

Bake for 18-24 minutes, until slightly crusty on the surface and deep golden brown on the bottom.

Rotate pans halfway through baking for even cooking.

Cool completely on racks before storing in an airtight container for up to 5 days.

For Cardamom Pannelets: Omit lemon zest and add cardamom, orange zest, and orange flower water to yam mixture.

For Masala Pannelets: Omit lemon zest and add garam masala to yam mixture. Roll in plain or cinnamon sugar.