Special equipment: an ebelskiver pan and a wooden skewer
For the herb buttermilk dip: Smash the garlic, sprinkle with salt and work to a paste with your knife blade. In a bowl, whisk together the yogurt, buttermilk, lime zest and juice, cilantro, parsley, garlic paste and some salt and pepper. Cover, and refrigerate for at least 30 minutes and up to 2 hours to allow the flavors to meld.
For the ebelskivers: In a large bowl, whisk together the flour, baking powder, baking soda, sea salt, nutmeg, black pepper and cayenne.
In a medium bowl, combine the buttermilk, egg yolks, sweet potato puree, 4 tablespoons melted butter and green onions. Whisk until smooth. Slowly add the sweet potato mixture to the dry ingredients, stirring until combined. Whip the egg whites in a medium bowl until stiff peaks form. Gently fold the egg whites into the batter.
Set the ebelskiver pan over medium heat, and brush each divot with melted butter. Heat until the butter begins to bubble. Add 2 tablespoons of the batter to each divot. Cook until the sides of the ebelskivers are golden brown and come away easily from the pan (use a skewer to check) and the top starts to dry a bit and bubble, 3 to 4 minutes. With a skewer, carefully turn the ebelskivers over and cook until deep golden brown on the second side, about 3 minutes more; transfer to a platter. Repeat with the remaining batter. Serve the ebelskivers with herb buttermilk dip on the side.