Preheat your oven to 350 degrees.

Combine flour, crushed fruity pebbles, sugar, baking powder, and salt in a large bowl.

In another bowl, whisk together milk, egg, vinegar, vanilla, and vegetable oil. Combine with flour mixture until a smooth batter forms.

Spray your mini doughnut pan with non-stick spray. Use an icing bag to pour the batter into the pan cavities. Fill each hole about halfway with batter.

Bake for 4 minutes or until they spring back when gently poked with your finger. Insert a toothpick and pull out to see if they're done. (If toothpick is clean, they're ready.) Let doughnuts cool while you prepare the frosting.

Whisk together powdered sugar, butter, and vanilla until frosting comes together, adding a splash of milk as needed to loosen it if it's too thick.

Dip half of the doughnut into the frosting, then cover with fruity pebbles cereal. Let sit on cooling rack until frosting stiffens up.