Preheat the oven to 425 and prepare a jumbo muffin tin with liners. Set to the side.
Shred the zucchini using a box grater. Set to the side.
In a large bowl, combine the sugar, eggs, oil, sour cream, vanilla, and melted butter. Whisk to combine. Then, add the buckwheat flour, AP flour, baking soda, and baking powder. Mix until there are no streaks left. Loosely fold in the zucchini.
Fill the prepared muffin tin with batter, filling each one up ¾th of the way full.
Bake the muffins at 425°F for 5 minutes, then turn the heat down to 350°F while they are in the oven. Continue to bake for 25-30 minutes, until the muffins are fully browned on top and a toothpick can run clean through them.
Let muffins slightly cool for about 10 minutes in the pan before taking them out. Eat them fresh for the best bite!