Preheat the oven to 350°F/176°C and spray your donut pan with baking spray. This recipe makes 12-15 donuts, so have more pans handy.
Add the cooled melted butter, oil, eggs, vanilla extract, and buttermilk to a mixing bowl and whisk until well combined.
In a large mixing bowl, mix the flour, granulated sugar, baking powder, baking soda, nutmeg, and salt until well combined.
Add the wet ingredients to the dry and mix until smooth and combined, being careful not to over mix the batter. The mixture will be pretty thick, but all of the flour should be incorporated. If the dough seems stiff or dry, add a little more buttermilk.
Pipe the dough evenly into the pans, filling the cavities ¾ full.
Bake 12 to 14 minutes or until a cake tester comes out clean.
Remove from the oven and let cool in the pan for 10 minutes. Then transfer to a cooling rack to cool for 15-20 minutes.
For the Frosting
Sift together the sugar, cocoa powder, and salt in a medium mixing bowl.
Add the milk, then mix the vanilla and corn syrup to make a smooth glaze. If the frosting is thick, add a little extra milk to loosen it to a pourable consistency.
Once the donuts are cooled, use a spoon to pour over the donuts or dip them into the bowl one at a time.
Allow the frosting to set before serving.
Notes
Correctly measure flour by spooning the flour into the measuring cup and then leveling off the top, or for most consistent results, use a kitchen scale.
Don’t overmix the batter when you combine the wet and dry ingredients. Overmixing can cause the donuts to be dense and not light and fluffy.
Use room temperature ingredients as they blend better when they’re all at the same temperature.
To quickly fill the pan, pipe the dough into the pan using a piping bag, or use a gallon-sized ziplock bag. Just cut a corner off the bottom of the bag and pipe the batter into each cavity.
Chocolate frosted donuts are best when eaten on the day they are baked.