Spray your doughnut pan/s with cooking spray and set aside.
In a large mixing bowl, sift the flour, cornstarch, sugar, cinnamon, baking powder and baking soda. Whisk the dry ingredients together until well combined.
In a separate bowl, whisk together your wet ingredients: milk, eggs, melted butter and vanilla extract.
Pour the wet ingredients into the bowl with the dry ingredients and mix with a spoon until well blended to make your batter.
Pour batter into your greased doughnut pan carefully with either a spoon or ladle or using an optional piping bag.
Bake for 8 to 10 minutes, or until the top of the doughnuts spring back when touched.
Allow your doughnuts to cool completely on a wire rack before adding glaze.
While the donuts are cooling on the wire rack, prepare the glaze by combining powdered sugar and milk into a mixing bowl. Whisk until ingredients are combined and a glaze forms, ensuring there are no lumps of powdered sugar left so that the glaze covers the doughnuts smoothly.
Doing only a couple at a time, dip each doughnut into the glaze to cover the top half, then sprinkle the purple, green, and yellow sanding sugars over the glaze.
Allow the glaze to dry and the doughnuts to set before serving. Enjoy!