Spray a donut pan with nonstick spray. Set it aside. Add the flour, crushed cereal, baking powder, cocoa, and salt to a large mixing bowl and whisk to combine.
Create a well in the center of the dry ingredients and add the sugar, melted butter, egg, vanilla extract, and buttermilk.
Start by whisking the wet ingredients together and gradually combine all the ingredients. The mixture will be thick and slightly lumpy. Using a rubber spatula, fold in Reese's puff cereal.
Use a pastry bag to pipe the batter equally into the six donut cavities.
Set the donut pan into the Air Fryer, set the temperature to 320 degrees F, air fryer setting, for 9-10 minutes until the donut bounces back when lightly touched. Turn them out onto a wire rack to let them cool. Repeat with any remaining batter.
Add all ingredients to a saucepan on medium-low heat. Use a rubber spatula constantly stir until the glaze becomes uniform and smooth.
Once the donuts are cool enough to handle, dip them into the glaze, place them back on the wire rack, and top with extra cereal, chocolate drizzle, and Reese's cups as desired.
Plate, serve, and enjoy!