Peel the sweetpotatoes and cut into large chunks. Place in a medium pot, season with a pinch of salt, and cover with water. Bring to a boil over medium-high heat and cook 10-12 minutes, until tender throughout when pierced with a fork. Remove from the heat, drain the water, and mash the potatoes. Set aside to cool to room temperature.
While the potatoes cool, make the honey syrup. In a small saucepan, combine the honey with ½ of the juice and zest of the orange, and add in a cinnamon stick. Warm over medium heat until just starting to bubble, then lower the heat and keep warm.
In a medium mixing bowl, sift together the flour, baking powder, and baking soda. Use a whisk to combine, then add in the brown sugar, 1 tsp salt, cinnamon, and nutmeg. Set aside.
In a large mixing bowl, combine the melted butter, buttermilk, whole milk, and eggs. Whisk in the mashed sweetpotatoes. Add the dry ingredients, gently folding together until just combined with no clumps or streaks.
Heat a waffle iron to medium-high heat. Once ready, spray with cooking spray, and ladle in a scant half-cup (120 ml) of the batter. Close and cook 5-7 minutes, or until the waffle iron indicates that it’s done. Place each waffle on a wire rack to cool and continue cooking until all the batter has been used.
To serve, stack the waffles, melt a pat of butter over the top, and drizzle over the honey syrup. Top with the toasted walnuts, mandarin wedges, and the remaining orange zest, and enjoy!