Preheat the oven to 325°F. Line muffins tin(s) with 24 liners.
Sift flour and cocoa powder together in a bowl and set aside.
In a large mixing bowl, beat together the butter, oil, sugar, vanilla, baking soda, baking powder, cinnamon, nutmeg and salt until smooth.
Beat in the eggs one at a time.
Mix in both the buttermilk and flour/cocoa mixture by alternating each.
Mix until smooth.
Fold in the zucchini and chocolate chips.
Divide batter evenly into the muffin tins.
Bake 20-25 minutes or until toothpick inserted comes out clean.
BUTTERCREAM:
Mix butter on medium speed until pale yellow.
With mixer on low speed slowly add in the powdered sugar alternating with milk. (if you find the buttercream to be too stiff add a little more milk as needed.)
Add in the vanilla and mix until fully combined.
ASSEMBLY:
When cupcakes are cooled pipe rosettes using Wilton tip #2D
Sprinkle with sprinkles.