For the cupcakes:
2 cups all-purpose flour
2 tsp. baking powder
½ tsp. baking soda
¼ tsp. salt
1 tsp. ground cinnamon
16 tbsp. unsalted butter, softened
1½ cups lightly packed brown sugar
3 large eggs
2 cups sweet potato puree*
6 tbsp. maple syrup
½ tsp. vanilla extract
Preheat the oven to 350° F. Line cupcake pans with paper liners. In a medium bowl combine the flour, baking powder, baking soda, salt and cinnamon; stir together with a fork and set aside. In the bowl of an electric mixer fitted with the paddle attachment, cream together the butter and sugar on medium-high speed until light and fluffy, about 2 minutes. Beat in the eggs one at a time, scraping down the bowl after each addition. Mix in the sweet potatoes, maple syrup and vanilla extract, beating just until combined. Reduce the mixer speed to low and mix in the dry ingredients, beating just until incorporated.
Divide the batter evenly between the prepared cupcake liners. Bake, rotating the pans halfway through, until a toothpick inserted in the center comes out clean, for about 18 minutes. Let cool in the pan briefly, then transfer to a wire rack to cool completely before frosting.
To make the frosting, combine the egg whites, sugar and cream of tartar in a bowl set over a pan of a few inches of simmering water. (Note: I use my clean, dry mixer bowl right off the bat to avoid dirtying two bowls.) Heat the mixture, whisking frequently, until it reaches 160° F with an instant-read thermometer. Transfer the mixture to an electric mixer fitted with the whisk attachment. Whisk starting at low speed and gradually increasing to medium-high speed until stiff, glossy peaks form. Mix in the vanilla until combined.
Transfer the frosting to a pastry bag fitted with a decorative tip (I used Wilton #21.) Pipe swirls of frosting on top of each cupcake. Brown the frosting with a kitchen torch**. Sprinkle lightly with additional brown sugar, if desired.
*To make the sweet potatoes, I baked sweet potatoes wrapped in foil at 400˚ F until fork-tender through to the center. Once cool enough to handle, I scooped out the flesh and mashed it until smooth.
**People often ask if there is an alternative method to brown the frosting without a kitchen torch. Is there? Yes. Do I recommend it? No. If you absolutely must, you can use the broiler in your oven. However, this offers far less control than a torch and it is a short moment of distraction can easily result in ruined frosting. Kitchen torches are inexpensive and a fun and handy tool to have around. If it is feasible, I highly recommend investing in one.