Place the flour, cocoa powder, cream of tartar, sugar, and yeast in the bowl of a stand mixer, and use a handheld whisk to combine well. Add the salt, and whisk to combine. Add the butter, oil and milk, then attach the dough hook to the stand mixer, and mix on low speed until combined. Raise the mixer speed to medium and mix for about 5 minutes. The dough should be smooth and stretchy. Spray a silicone spatula lightly with cooking oil spray, and scrape down the sides of the bowl. Transfer the dough to a lightly oiled bowl or proofing bucket large enough for the dough to rise to double its size, and cover with an oiled piece of plastic wrap (or the oiled top of your proofing bucket). Place the dough in the refrigerator for at least 12 hours and up to 5 days. (See Recipe Notes)

Once the dough has undergone its first rise, turn it out onto a lightly floured surface and, using the scrape and fold kneading method and using a very light touch, sprinkle the dough with more flour and knead it lightly, sprinkling with flour when necessary to prevent it from sticking, scrape the dough off the floured surface with a floured bench scraper, then fold it over on itself. Repeat scraping and folding until the dough has become smoother. Do not overwork the dough or you will incorporate too much flour and it will not rise properly. Roll out the dough on a lightly floured surface into a rectangle about 1-inch thick, sprinkling very lightly with flour to prevent sticking. With a floured round cookie cutter about 1-inch in diameter, cut out rounds of dough and place each piece on a greased, parchment-lined baking sheet. Cover with lightly oiled plastic wrap, and set in a warm, draft-free location to rise for about 45 minutes, or until nearly doubled in size.

As the dough is nearing the end of its rise, place about 3-inches of frying oil in a medium-size, heavy-bottom pot or fryer. Clip a deep-fry/candy thermometer to the side of the pot or fryer, and place the oil over medium-high heat. Bring the oil temperature to 350°F. Place the raised donut holes a few at a time in the hot oil, taking care not to crowd the oil. Fry for about 2 minutes per side or until they are brown all over. When the donuts first begin to fry, they will become lighter in color, and then will brown. Remove them before they become very dark. As soon as each batch is removed from the fryer, place on a wire rack placed over paper towels to drain and cool completely.

In a small bowl, place the confectioners’ sugar, cocoa powder, maple syrup and 3 tablespoons milk. Mix well, until a thick paste forms. Add more milk by the 1/4-teaspoon, mixing to combine well, until the glaze falls off the spoon slowly, in a thick but pourable glaze. Add milk very slowly, as it is much easier to thin, than to thicken, the glaze. If you do thin the glaze too much, add more confectioners’ sugar a teaspoon at a time to thicken it.

Immerse each cooled donut hole in the glaze and lift out with the tines of a fork or chocolate dipping tool, and return to the wire rack for any excess glaze to drip off. Allow the glaze to set at room temperature and serve immediately.