Pre-heat oven to 350 degrees and lightly grease 6-cavity doughnut tin with cooking spray.
In the bowl of a stand mixer or in a large bowl with a hand mixer, beat together butter and sugar until light and fluffy. Add the egg and vanilla extract and mix to combine.
In a separate bowl, whisk together flour, baking powder, and salt.
Pour dry ingredients into the butter/sugar, alternating with milk, and mix until just combined. Take care not to over-mix or doughnuts will be dense.
Fold ½ cup Fruity Pebbles into the batter.
Evenly distribute batter amongst cavities of prepared doughnut pan, filling tin almost all the way.
Bake for 15 minutes, until the doughnuts are starting to turn golden and spring back when touched. Let cool in pan for a few minutes, before moving to wire rack to cool completely before icing.
When doughnuts are cooled, dip upside down into icing to coat the tops. Sprinkle remaining cup of Fruity Pebbles over icing.