Preheat oven to 350°F and grease a donut pan.
In a bowl, whisk together the flour, sugar, baking powder, baking soda, and salt.
In another bowl, mix the buttermilk, egg, melted butter, and vanilla extract.
Combine the wet and dry ingredients until just mixed, then fold in the Fruity Pebbles cereal.
Spoon the batter into the donut pan, filling each cavity about 2/3 full.
Bake for 10-12 minutes or until a toothpick comes out clean.
Let the donuts cool in the pan for 5 minutes, then transfer to a wire rack.
For the glaze, mix powdered sugar, milk, and vanilla until smooth.
Dip cooled donuts into the glaze, then sprinkle with extra Fruity Pebbles cereal.