1. Preheat an oven to 400°F
2. Bring a large pot of water to a boil over medium-high heat.
3. Add the sweet potatoes and cook until tender, 15 to 20 minutes.
4. Remove from heat, drain well.
5. Transfer to a food processor and pulse until slightly fluffy.
6. Spoon the sweet potatoes into a bowl. Cool to room temperature.
7. Grease 12 regular size muffin cups with butter. Place cups in muffin tins.
8. In a bowl, sift together the flour, cinnamon, nutmeg, orange zest, baking powder and salt.
9. With your stand mixer, fixed with the whisk attachment, whisk the eggs and sugar on high for 30 seconds.
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11. Slowly add butter in two or three pours, whisking to combine after each.
12. Add sour cream and sweet potatoes, whisk until just combined.
13. With mixer on low slowly fold in the flour mixture in 2 pours. Mix until just evenly moistened.
14. The batter will be slightly lumpy.

Optional:

1. Using a large rubber spatula, fold in the craisins and pecans until just evenly distributed, Do not overmix.
2. Spoon the batter into the prepared muffin cups, filling them three-fourths full.
3. Bake until the muffins are golden and a toothpick inserted into the center comes out clean, 18 to 22 minutes.
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5. Transfer the pan to a wire rack and let cool for 5 minutes.

Topping:

1. While muffins are baking. combine sugar and cinnamon in a bowl.
2. Melt 4 tablespoons of butter in another bowl..
3. After the muffins have cooled for 5 minutes, working with one muffin at a time, dip the top of the muffin in melted butter then into the cinnamon/sugar mixture.
4. Place on baking rack to cool.
5. Serve warm.