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 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 gently until just combined; avoid overmixing to prevent tough donuts.
05 - Combine melted butter, brown sugar, and cinnamon in a small bowl. Stir until sugar is completely dissolved and mixture forms a paste.
06 - Spoon half the donut batter evenly into the prepared pan cavities, filling each about one-third full. Add approximately 1 teaspoon of cinnamon swirl mixture on top of each, then cover with remaining batter.
07 - Gently swirl a toothpick through the batter in each cavity 2-3 times to distribute the cinnamon filling, being careful not to touch the pan bottom.
08 - Bake for 10–12 minutes, or until donuts spring back when lightly pressed and edges are golden brown.
09 - Let donuts cool in pan for 5 minutes, then carefully transfer to a wire rack to cool completely before glazing.
10 - Whisk powdered sugar, milk, and vanilla in a medium bowl until smooth and pourable. Add additional milk 1 teaspoon at a time if glaze is too thick.
11 - 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 traditional King Cake stripes.
12 - Let glaze set for 10 minutes at room temperature before serving. Best enjoyed the same day but can be stored in an airtight container for up to 2 days.