In a large bowl, whisk together the flour, finely crushed cereal, cocoa powder, baking powder, and salt.

Create a well in the center and add the sugar, melted butter, egg, vanilla, and buttermilk.

Whisk the wet ingredients together in the center, then gradually incorporate the dry ingredients. The batter will be thick and slightly lumpy — do not overmix.

Fold in the whole cereal with a rubber spatula.

Pipe the batter into the donut cavities, filling each about three-quarters full.

Place the pan in the air fryer and cook at 320°F for 9–10 minutes, until the donuts spring back when lightly pressed. Turn out onto a wire rack to cool. Repeat with remaining batter.

For the glaze, combine the confectioners' sugar, milk, and vanilla in a small saucepan over medium-low heat. Stir constantly until smooth.

Dip the cooled donuts into the glaze, return them to the wire rack, and top immediately with cereal, chopped Reese's cups, and chocolate drizzle.