01 - Preheat oven to 350°F. Grease a 12-cavity donut pan thoroughly with cooking spray or butter.
02 - In a large bowl, whisk together flour, granulated sugar, baking powder, cinnamon, and salt until evenly combined.
03 - In a separate bowl, whisk milk, eggs, melted butter, and vanilla extract until the mixture is smooth and well blended.
04 - Pour wet ingredients into dry ingredients. Mix gently with a spatula just until combined; do not overmix to ensure tender donuts.
05 - In a small bowl, mix softened butter, brown sugar, and cinnamon until a smooth paste forms.
06 - Spoon half the batter into prepared donut pan cavities. Pipe or dot a ring of cinnamon swirl mixture onto each. Cover with remaining batter.
07 - Bake for 10-12 minutes until donuts spring back when lightly touched. Cool in pan for 5 minutes, then transfer to wire rack.
08 - Whisk powdered sugar, milk, and vanilla until smooth and pourable. Add more milk if too thick.
09 - Dip cooled donuts into glaze, then immediately top with purple, green, and gold sprinkles. Let set before serving.