In a medium bowl, whisk together the flour, baking powder, baking soda, salt, and cinnamon.
Using the large holes of a box grater, grate the apples into a bowl. (No need to peel them unless you want to.) Squeeze the grated apples firmly to remove as much juice as possible. Separate and loosen the compressed apple shreds, then measure 2/3 cup packed apple shreds (90 g) and 3 tablespoons apple juice (44 g). Combine the measured shreds and juice and discard any excess.
In a large bowl, whisk together the reserved grated apples, apple juice, apple cider concentrate, brown sugar, granulated sugar, melted butter, eggs, and sour cream until well combined.
Add the dry ingredients and fold with a rubber spatula until just combined.
Cover the bowl with plastic wrap and refrigerate for 3–4 hours. The dough can be refrigerated overnight, but the donuts will have the softest, most tender texture when fried the same day.
Fry the Donuts
Heat 3-4 inches of neutral oil in a Dutch oven or heavy-bottomed pot to 365°F (185°C).
Line a baking sheet with a wire rack. Cut 11 pieces of parchment paper into roughly 4×4-inch squares and arrange them on a second baking sheet.
Turn the chilled dough onto a generously floured surface. Flour the top, flip the dough over, and flour the other side. Roll to 1/2-inch thickness.
Using a floured 3-inch donut cutter, cut out as many donuts as possible, pressing firmly and twisting the cutter back and forth a few times to cut through the apple shreds, and re-flouring the cutter between cuts. Then, transfer each donut to its own parchment square.
Gather the scraps, reroll to 1/2-inch thickness, and continue cutting donuts. Lightly brush away any excess flour from the donuts if needed.
Working in batches, use a spider or slotted spoon to carefully lower 2 donuts at a time into the hot oil. Fry for 3 minutes total, flipping halfway through.
Transfer the donuts to the prepared wire rack. Allow the oil to return to 365°F (185°C) before frying the next batch.
Coat in Apple Cinnamon Sugar
In a shallow bowl, stir together the sugar, freeze-dried apple powder, and cinnamon.
Once the donuts have cooled completely, lightly brush each one with the remaining apple cider concentrate, then immediately toss in the apple cinnamon sugar until evenly coated. Enjoy!
Notes
*Measure your dry ingredients properly. This is my #1 baking tip! Never scoop flour into your measuring cup, as this always leads to using too much flour. Instead, use the spoon-level method. This means fluffing the flour first, then spooning it into your measuring cup/spoon. For the BEST results, use a kitchen scale!*Notes on donut cutter: If you don't have a donut cutter, use a 3-inch round cutter to cut the dough and transfer each round to a parchment square. Use a 1-inch round cutter to cut out the centers.Notes on apple powder: I tested the donuts with and without the apple powder. Both versions have plenty of apple flavor, but adding it to the cinnamon sugar gives them a much more intense, concentrated apple flavor. Feel free to leave it out for a more subtle apple flavor. To make freeze-dried apple powder, finely grind freeze-dried apples (I bought some at Walmart) in a dry blender or spice grinder until powdery.Storage: Freshly fried donuts are always best enjoyed the day they are made, but they keep surprisingly well the next day. Store them in a single layer on a baking sheet, uncovered or loosely covered with foil. Avoid sealing them in an airtight container, as trapped moisture will cause the donuts to become soft and soggy.Frying tips: Use at least 3–4 inches of oil. A larger volume of oil holds its temperature more steadily when the donuts are added, helping them cook evenly and preventing them from absorbing excess oil.