01 - Preheat oven to 350°F. Grease a 12-cavity donut pan thoroughly 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 evenly distributed.
03 - In a separate bowl, whisk whole milk, eggs, melted unsalted butter, and vanilla extract until fully incorporated.
04 - Pour wet ingredients into dry ingredients. Stir gently until just combined; avoid overmixing to prevent tough donuts.
05 - Combine melted butter, packed light brown sugar, and ground cinnamon in a small bowl. Mix until smooth and spreadable.
06 - Spoon half the donut batter evenly into the prepared donut pan cavities, filling each about one-third full.
07 - Place approximately 1 teaspoon of cinnamon swirl mixture on top of the batter in each cavity. Cover with remaining batter.
08 - Gently swirl a toothpick through the batter in each cavity to create marble patterns, being careful not to touch the pan sides.
09 - Bake for 10–12 minutes, or until donuts spring back when lightly pressed and edges are golden brown.
10 - Let donuts cool in the pan for 5 minutes, then carefully transfer to a wire rack to cool completely before glazing.
11 - Whisk powdered sugar, milk, and vanilla extract in a medium bowl until smooth and pourable. Adjust milk consistency as needed.
12 - Dip the top of each cooled donut into the glaze, allowing excess to drip off. Immediately decorate with purple, green, and yellow sanding sugars in sections to mimic traditional King Cake stripes.
13 - Let glaze set for approximately 10 minutes before serving. Donuts are best enjoyed the same day but can be stored in an airtight container.