1cupmilk with 2 teaspoons white vinegar stirred in
Some oil for lightly spraying
Preheat the oven to 425 F. Lightly grease a donut pan.
Combine the flours, starch, baking powder, baking soda, cinnamon salt and xanthan gum in a bowl.
In a separate, larger bowl, beat the butter, oil and sugars until combined, then beat in the eggs and vanilla extract. Alternate beating in half of the milk, then half of the flour mixture followed by the remaining milk and flour mixture. Beat just until combined.
Scoop the batter into the donut molds, filling them to the top.
Bake the donuts on the middle rack for 7-8 minutes until golden and a toothpick inserted into the middle comes out clean. Let the donuts remain in the pan for 2 minutes before removing them.
I like to give the donuts a very light spray of oil before I roll them in the cinnamon sugar so that better adheres to the donuts. You can do that while they're still warm or once they've cooled.