Combine flour, sugar, baking powder and salt and mix.
Using a pastry cutter cut butter into your flour mixture until it resembles coarse crumbs.
Add milk and mix until all ingredients are combined.
Transfer dough onto a well-floured surface, and knead gently until it forms a cohesive ball. If dough is too sticky to manage, continue to work in flour until it is smooth and manageable (I've found on hot days or if my butter isn't cold enough the dough can be quite sticky)
Break off approximately 1 ½ Tbsp-sized pieces of dough and roll into smooth, tight balls. Set aside.
Fill a medium-sized saucepan 2-inches deep with your oil over medium-high heat. Use a candy thermometer (make sure it isn't touching the bottom of the pot) to monitor your temperature.
Heat oil to 350F (you will have to monitor the heat pretty regularly and may need to increase/decrease your stove temperature to keep it consistent).
While your oil is heating, prepare your cinnamon/sugar mix by stirring together cinnamon and sugar in a small dish. Set aside.
Prepare two plates for your cooked donut holes by lining them generously with paper towels.
Once oil has reached 350F, very carefully fry your donut holes, about 4 at a time, carefully transferring them to the oil with a slotted spoon (don't drop them in or the oil may splash, lower them into the oil instead).
Fry donut holes for approximately 3.5 minutes*, remove carefully with a slotted spoon, and place them on a paper towel laden plate. Allow them to sit for about 30 seconds and then use another spoon to transfer them to your cinnamon/sugar dish, roll them in the topping until fully covered, and then transfer to your other paper towel lined plate.
Repeat until all donut holes are cooked and have been rolled in cinnamon sugar.
These donuts are best enjoyed warm/hot, and do not carry over well to the next day.
*For best results, fry only one donut hole the first time you try this, and gauge how thoroughly it is cooked. After 3.5 minutes has passed, cut into it to check whether it is underdone, just right, or overdone, and then fry the rest of your donut holes accordingly. While these should take about 3.5 minutes under optimal conditions with a consistent oil temperature of 350F, it's still important to check so that you don't end up with 1 ½ dozen donuts that are still gooey in the middle!