Preheat the oven to 375 degrees.

In a large mixing bowl, cream together the butter and the sugar. Add the sour cream, buttermilk, and zucchini. Mix on low, scraping down the sides of the bowl as needed.

Measure the dry ingredients in a small mixing bowl and stir lightly with a fork to combine.

Pour the dry ingredients into the buttermilk mixture and beat for 1-2 minutes on low speed.

Stir in the chocolate chips by hand.

Spoon the batter into a large ziplock bag and seal. Snip off one corner and pipe the donut batter into a prepared donut pan, filling each cavity 1/2 full.

Bake at 375 degrees for 10-15 minutes or until a toothpick inserted in the thickest part of the donut comes out clean.

Cool in the pan for 10 minutes and then turn out. Cool completely before glazing.