01 - Preheat oven to 350°F and generously grease a 12-cavity donut pan with cooking spray or butter.
02 - In a large mixing bowl, whisk together flour, granulated sugar, baking powder, salt, and 1 teaspoon ground cinnamon until well combined.
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; the batter should remain slightly thick. Do not overmix.
05 - In a small bowl, combine 2 tablespoons melted butter, brown sugar, and 1 tablespoon cinnamon. Stir until mixture forms a paste-like consistency.
06 - Spoon half the batter evenly into donut pan cavities, filling each about one-third full. Add 1 teaspoon cinnamon swirl mixture atop each cavity.
07 - Cover cinnamon filling with remaining batter, dividing evenly among cavities. Gently swirl with a toothpick to create marble pattern without over-mixing.
08 - Bake for 10-12 minutes until donuts spring back when lightly pressed and edges are golden brown. Avoid overbaking for optimal texture.
09 - Let donuts rest in pan for 5 minutes before transferring to wire rack. Allow to cool fully before glazing to prevent melting.
10 - Whisk powdered sugar, 2 tablespoons milk, and vanilla extract until smooth. Add additional milk 1 teaspoon at a time if needed for pourable consistency.
11 - Dip tops of cooled donuts into glaze, allowing excess to drip off. Immediately sprinkle sections with purple, green, and yellow sanding sugars in traditional King Cake pattern.
12 - Let glaze set for 10 minutes at room temperature before serving. Donuts are best enjoyed the same day but can be stored in an airtight container.