Preheat oven to 400°F. Spray donut tins well with nonstick spray. Set aside. For 18 donuts, I used donut tins that make 3-inch-sized donuts.
In a medium bowl, whisk the flour, baking powder, baking soda, salt, and nutmeg together. Set aside.
2½ cups all-purpose flour, 2 teaspoons baking powder, ½ teaspoon baking soda, ½ teaspoon kosher salt, ¼ teaspoon ground nutmeg
In a large bowl, using a hand mixer, beat the butter, vegetable oil, and sugar together on high until well mixed, about 1 minute.
¼ cup unsalted butter, ¼ cup vegetable oil, 1 cup granulated sugar
Beat the eggs and vanilla extract into the wet ingredients on high speed until well mixed, about 1 minute.
2 large eggs, 2 teaspoons pure vanilla extract
On a low setting, beat half of the flour mixture into the wet ingredients just until combined. Do not overmix.
Add in all of the buttermilk and beat on low until combined.
¾ cup buttermilk
Add in the remaining flour mixture and beat on low just until fully combined. It’s okay if the batter is still slightly lumpy. Do not overmix or the donuts won’t bake up fluffy.
Gently fold in the mini chocolate chips until mixed well.
1 cup mini chocolate chips
Place the donut batter into a large piping bag and pipe the batter into the wells of the donut tins. Fill each donut well up only ¾ of the way full so they have room to grow without overflowing.
Bake in the oven for 15-16 minutes, or until a toothpick inserted in a donut comes out clean.
Cool for 15 minutes in the tin before removing from the tin to cool completely. Let cool completely before frosting.