Set up the fry station. Pour 2 inches of oil into a heavy pot or Dutch oven. Clip on a thermometer if you have one. Line a baking sheet with paper towels and place a cooling rack on top for draining.

Heat the oil. Warm the oil to 350°F (175°C). Keep the heat steady. You’ll adjust as you fry to stay near that temperature.

Mix the dry ingredients. In a large bowl, whisk 2 cups all-purpose flour, 1/3 cup granulated sugar, 2 teaspoons baking powder, 1/2 teaspoon kosher salt, and 1/2 teaspoon ground cinnamon.

Whisk the wet ingredients. In a separate bowl, whisk 3/4 cup milk or buttermilk, 1 large egg, 3 tablespoons melted unsalted butter (slightly cooled), 1 teaspoon vanilla extract, and 2 tablespoons brown sugar if using.

Combine gently. Pour the wet ingredients into the dry and stir with a spatula until just combined. The batter will be thick and sticky. Do not overmix—a few small lumps are fine.

Make the cinnamon sugar. In a shallow bowl, mix 1/2 cup granulated sugar with 1 to 2 teaspoons ground cinnamon. Set aside.

Fry the doughnut holes. Using a small cookie scoop or two spoons, drop tablespoon-sized mounds into the hot oil. Fry in batches to avoid crowding. Cook for 2 to 3 minutes, turning once, until deep golden and puffed.

Drain and coat. Transfer to the rack for 30 seconds to let excess oil drip, then roll in the cinnamon sugar while still warm. Work in small batches so the sugar sticks well.

Adjust heat as needed. If the doughnut holes brown too fast, lower the heat; if they take too long or get greasy, raise it slightly. Keep the oil near 350°F for best results.

Serve warm. They’re best fresh, but still tasty at room temperature. Add an extra pinch of cinnamon sugar right before serving if you like.