01 - Preheat oven to 350°F. Grease a 12-cavity donut pan thoroughly.
02 - In a large bowl, whisk together flour, granulated sugar, baking powder, cinnamon, and salt until evenly distributed.
03 - In a separate bowl, whisk milk, eggs, melted butter, and vanilla extract until well combined and smooth.
04 - Add wet ingredients to dry ingredients and mix gently until just combined. Do not overmix to maintain tender texture.
05 - In a small bowl, mix softened butter, brown sugar, and cinnamon until a uniform paste forms.
06 - Spoon half the donut batter into the prepared donut pan. Pipe or dot a ring of cinnamon swirl mixture onto each donut. Top with remaining batter to completely cover the swirl layer.
07 - Bake for 10-12 minutes or until donuts spring back when lightly pressed. Cool in pan for 5 minutes, then transfer to wire rack to cool completely.
08 - Whisk powdered sugar, milk, and vanilla until smooth and pourable. Adjust consistency with additional milk if needed.
09 - Dip cooled donuts into the glaze, then immediately decorate with alternating purple, green, and gold sprinkles. Allow glaze to set before serving.