Whisk together the all-purpose flour, baking powder, salt, and monk fruit sweetener in a large mixing bowl to ensure even leavening distribution.

Incorporate the Greek yogurt and vanilla extract into the dry mixture using a spatula, stirring until a shaggy dough begins to form.

Fold in the protein powder last; this technical step prevents the whey from becoming excessively sticky during the initial hydration phase.

Transfer the dough to a lightly floured surface and knead for approximately 60 seconds until a smooth, cohesive ball is formed.

Segment the dough into 12 equal portions, approximately 28 grams each.

Roll each portion between the palms to create uniform spheres, ensuring no deep cracks remain on the surface.

Preheat the air fryer to 350°F (175°C).

Arrange the donut holes in the air fryer basket in a single layer, maintaining at least 15mm of clearance between each unit to facilitate optimal airflow.

Air fry for 7 to 8 minutes, shaking the basket at the 4-minute mark to ensure even browning across all surfaces.

Remove the donut holes and immediately brush with a thin layer of melted butter while the surface temperature remains high.

Roll each hole in the cinnamon-erythritol blend until completely coated, then transfer to a wire rack to cool slightly before serving.