01 - Preheat oven to 350°F. Grease a 12-cavity donut pan thoroughly with cooking spray or butter.
02 - In a large mixing bowl, whisk together flour, granulated sugar, baking powder, salt, and ground cinnamon until well blended.
03 - In a separate bowl, whisk milk, eggs, melted butter, and vanilla extract until smooth and fully incorporated.
04 - Pour wet ingredients into dry ingredients. Stir gently until just combined; avoid overmixing to prevent tough donuts.
05 - In a small bowl, combine melted butter, brown sugar, and cinnamon. Stir until mixture forms a smooth paste.
06 - Spoon half the donut batter evenly into the prepared donut pan cavities, filling each about one-third full.
07 - Place about 1 teaspoon of cinnamon swirl mixture on top of the batter in each cavity. Cover with remaining batter.
08 - Gently swirl a toothpick through the batter in each cavity to create marbleized cinnamon pattern.
09 - Bake for 10-12 minutes until donuts spring back when lightly touched and edges are lightly golden.
10 - Let donuts cool in pan for 5 minutes, then carefully transfer to a wire rack to cool completely.
11 - Whisk powdered sugar, 2 tablespoons milk, and vanilla until smooth. Add additional milk if needed to reach pourable consistency.
12 - Dip cooled donuts into glaze, allowing excess to drip off. Immediately sprinkle with purple, green, and yellow sanding sugars in alternating sections.
13 - Let glaze set for 10 minutes before serving. Best enjoyed fresh but will keep for 1-2 days in an airtight container.