Heat oven to 350°F. Line a 12-cup muffin pan with paper liners
In a food processor, finely grind pecans. Add the flour, baking powder, baking soda, salt, cinnamon and ¼ teaspoon nutmeg and pulse to combine; set aside.
Using an electric mixer, beat ½ cup (1 stick) butter with the brown and granulated sugars and clementine zest until light and fluffy, 3 to 4 minutes. Beat in the eggs one at a time.
Reduce speed to low and gradually add the pecan mixture, mixing until just incorporated. Stir in the buttermilk and 1 teaspoon vanilla. Divide the batter among the muffin-pan cups.
Bake until golden brown and a wooden pick inserted into the center comes out clean, 12 to 18 minutes. Let cool for 10 minutes, then remove from pan and transfer to a wire rack to cool completely.
When ready to serve, make the frosting: In a large bowl, using an electric mixer, beat the confectioners’ sugar, 1 Tbsp cream and remaining 1/8 teaspoon nutmeg, teaspoon vanilla and 4 tablespoons butter until fluffy, adding another Tbsp cream if frosting is too thick. Spread on cupcakes and top with orange slices and pecans, if desired.