01 - Preheat oven to 375°F. Grease a 12-cavity donut pan thoroughly 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, vanilla extract, and lemon zest until fully incorporated.
04 - Pour wet ingredients into dry ingredients. Stir gently until just combined, being careful not to overmix the batter.
05 - In a small bowl, combine melted butter, brown sugar, and cinnamon. Stir until mixture forms a smooth paste.
06 - Spoon half the batter evenly into donut pan cavities. Create a small ring of cinnamon sugar filling in each cavity, then cover with remaining batter.
07 - Bake for 10-12 minutes until donuts spring back when lightly touched and edges are lightly golden.
08 - Let donuts cool in pan for 5 minutes, then carefully transfer to a wire rack to cool completely before glazing.
09 - Whisk powdered sugar, milk, and vanilla extract in a small bowl until smooth and pourable. Add more milk if needed for proper consistency.
10 - Dip tops of cooled donuts into glaze, then immediately sprinkle with purple, green, and gold sugars in alternating sections to create traditional King Cake stripes. Allow glaze to set before serving.