01 - Preheat oven to 350°F. Generously grease a 12-cavity donut pan with cooking spray or butter.
02 - In a large bowl, whisk together flour, granulated sugar, baking powder, cinnamon, and salt 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. Fold gently with a spatula just until combined; avoid overmixing to prevent tough donuts.
05 - In a small bowl, mix softened butter, brown sugar, and cinnamon until a smooth paste forms.
06 - Spoon half the batter into donut pan cavities, filling each about one-third full. Pipe or spoon a ring of cinnamon swirl mixture onto each donut, then top with remaining batter to completely cover the swirl.
07 - Bake for 10-12 minutes until donuts spring back when lightly touched and edges are golden brown. Cool in pan for 5 minutes before transferring to wire rack.
08 - Whisk powdered sugar, 2 tablespoons milk, and vanilla extract until smooth. Add additional milk, 1 teaspoon at a time, if needed to reach pourable consistency.
09 - Dip tops of cooled donuts into glaze, allowing excess to drip off. Immediately sprinkle with alternating purple, green, and gold sprinkles. Let glaze set for 15 minutes before serving.