In a microwave-safe container, heat the apple cider and butter to warm (120-130 degrees).
Add the vanilla to the butter and warm apple cider mixture. Set aside for just a moment.
In the bowl of a stand mixer (or large bowl, using a wooden spoon), combine 2 cups of flour, the dry yeast, sugar, salt, and cinnamon.
Add the apple cider mixture to the flour mixture and mix thoroughly.
Add the additional flour and the egg yolks, mixing continually until a soft dough forms (this will take 4-5 minutes with the stand mixer).
Remove the dough from the mixer and on a lightly floured surface, knead the dough by hand (for approximately 4-5 minutes) and then place the dough into a lightly greased bowl.
Cover the bowl with plastic wrap and allow the dough to rise until doubled (about an hour and a half) in a warm, draft-free place.
Once the dough has doubled in volume, punch down and allow to rest for 5-10 minutes.
Divide the dough in half. On a lightly floured surface, roll (or pat) the dough to ½" thickness.
Using a 3" biscuit cutter, cut the dough into circles, and then use a 1" dough cutter to add the center hole to the donuts. Reserve the holes to fry into donut holes.OR, use the 1" cutter to strictly make donut holes. Your call.
Allow your donuts to rise again, covered loosely with plastic wrap for approximately 45 minutes to an hour in a warm, draft-free place until they have doubled in size.
In a large deep-sided skillet (or deep fryer), heat roughly 2" of canola, avocado, or peanut oil to 350-degrees on an instant read thermometer.
Carefully use a large slotted spoon or spider to place the donuts 2 or 3 at a time into the hot oil and fry until they're golden brown on each side. Each side will take approximately 30-45 seconds. Watch them closely.
Using the slotted spoon, carefully remove the donuts from the hot oil and place them on a wire rack with paper towels underneath until they're completely cool.
To glaze the donuts:
In a medium bowl, whisk all the ingredients until smooth and slightly runny in consistency.
Dip the completely cooled donuts into the glaze, turning them once to coat the entire donut. Place the glazed donuts on a wire rack and allow the glaze to dry thoroughly before serving.
Notes
These donuts are best eaten the same day they're made.Store leftovers in a brown paper bag at room temperature for up to 2 days.Store leftovers in an airtight container in the refrigerator for up to 5 days.May be frozen. Store in an airtight freezer container or freezer bag. Label and date. Freeze for up to 2 months. Thaw in the refrigerator overnight.