01 - Preheat oven to 350°F. Generously grease a 12-cavity donut pan with cooking spray or butter.
02 - In a large mixing bowl, whisk together all-purpose flour, granulated sugar, baking powder, salt, and ground cinnamon until well combined.
03 - In a separate bowl, whisk whole milk, eggs, melted unsalted butter, and vanilla extract until smooth.
04 - Pour the wet ingredients into the dry ingredients. Stir gently until just combined, being careful not to overmix the batter.
05 - In a small bowl, mix melted butter, light brown sugar, and ground cinnamon until a smooth paste forms.
06 - Spoon half the batter evenly into the prepared donut pan cavities, filling each about one-third full.
07 - Place about 1 teaspoon of the cinnamon swirl mixture on top of the batter in each cavity, then cover with the remaining batter.
08 - Gently swirl a toothpick through each donut to create a marbled cinnamon pattern, being careful not to disturb the pan structure.
09 - Bake for 10-12 minutes until the donuts spring back when lightly touched and are lightly golden around the edges.
10 - Let the donuts cool in the pan for 5 minutes, then carefully transfer to a wire rack to cool completely before glazing.
11 - Whisk together powdered sugar, milk, and vanilla extract until smooth and pourable. Adjust consistency with additional milk if needed.
12 - Dip the top of each cooled donut into the glaze, allowing excess to drip off. Immediately sprinkle sections with purple, green, and yellow sanding sugars to create King Cake stripes.
13 - Let the glaze set for 10 minutes at room temperature before serving. Enjoy immediately or store in an airtight container.