01 - Preheat oven to 350°F. Grease a 12-cavity donut pan with cooking spray or butter.
02 - In a large 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 until just combined; do not overmix to maintain tender texture.
05 - Combine melted butter, brown sugar, and cinnamon in a small bowl. Mix until smooth paste forms.
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 each batter portion, then cover with remaining batter. Swirl gently with a toothpick to create marbled effect.
08 - Bake for 10–12 minutes until donuts spring back when lightly pressed and edges are golden brown.
09 - Let cool in pan for 5 minutes, then carefully transfer to wire rack to cool completely before glazing.
10 - Whisk powdered sugar, milk, and vanilla until smooth and pourable consistency. Add more milk if needed.
11 - Dip cooled donuts into glaze, letting excess drip off. Immediately sprinkle sections with purple, green, and yellow sanding sugars to create traditional King Cake stripes.
12 - Let glaze set for 10 minutes before serving to ensure decorations adhere properly.