• In a large bowl, whisk the flour, sugar, baking powder, salt, and cinnamon together.
• Add the butter and use a pastry cutter, two knives, or your fingers to work in the butter until it's the size of small peas.
• Pour in the milk and stir until everything is combined.
• Gently knead the dough until it comes together. It should be just sticky enough to hold together. Cover and refrigerate until you're ready to fry.
• If cooking over a campfire, prepare a hot fire. Pour enough oil into a small Dutch oven or pot to reach a depth of 2 inches. Attach a deep-fry thermometer to the side of the pot.
• Set it over the hot fire, or on a camp stove or stovetop over high, and heat to a temperature of 350°F.
• Pinch off rounds of dough and shape them into golf ball-sized balls. Work with the dough just long enough to create a ball; don't overwork it or your doughnut will be tough.
• Working with 3 or 4 at a time, add them to the oil and cook until golden brown all over, 3 to 3 ½ minutes. Check one after the first batch to determine the optimal cooking time for your dough. Use a slotted spoon or spider to carefully remove the doughnuts from the hot oil and transfer them to paper towels to drain.
• Repeat with the remaining dough, letting your oil return to temperature between batches.

Make the cinnamon sugar

• While the doughnuts are cooking, combine the sugar and cinnamon in a resealable bag or small bowl.
• Working with a couple at a time, add the warm cooked doughnuts to the cinnamon sugar and shake or roll to coat.
• Serve warm.