Preheat and prep. Preheat your air fryer to 350°F (175°C) for 3–5 minutes. Lightly mist the basket with oil or use a small sheet of air-fryer-safe perforated parchment.

Mix dry ingredients. In a large bowl, whisk flour, sugar, baking powder, baking soda, salt, and nutmeg until evenly combined.

Mix wet ingredients. In a separate bowl, whisk buttermilk, egg, melted butter, and vanilla until smooth. Make sure the butter isn’t hot, or it can scramble the egg.

Make the dough. Pour wet ingredients into dry. Stir gently with a spatula until just combined. The dough should be soft and slightly sticky. Avoid overmixing, which makes tough donuts.

Shape the donuts. Lightly flour your hands and a work surface. Pat the dough to about 1/2-inch thickness. Cut donuts with a donut cutter, gathering and re-rolling scraps as needed. You should get 8–10 donuts plus holes.

Lightly oil the tops. Place donuts on a tray or parchment and lightly mist or brush the tops with oil. This helps them brown evenly in the air fryer.

Air fry in batches. Arrange donuts in a single layer in the basket, leaving space between each. Air fry for 5–7 minutes, flipping halfway through, until golden and springy to the touch. Donut holes take about 3–5 minutes total.

Make the coating. While the donuts cook, mix sugar and cinnamon in a shallow bowl. Melt the 4 tbsp butter in a separate bowl.

Coat the donuts. Brush warm donuts with melted butter on all sides, then roll in the cinnamon sugar until fully coated. Coat as soon as they’re cool enough to handle so the sugar sticks well.

Serve warm. Enjoy immediately for the best texture—crisp edges and a soft center with a fragrant cinnamon finish.