01 - Preheat oven to 375°F and grease a 12-cavity donut pan with cooking spray or butter.
02 - In a large bowl, whisk together flour, granulated sugar, baking powder, salt, and ground cinnamon until well combined.
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 and stir until just combined. Be careful not to overmix the batter.
05 - In a small bowl, mix melted butter, brown sugar, and cinnamon until smooth and set aside.
06 - Spoon half of the batter evenly into the donut pan cavities. Create a small ring of cinnamon sugar filling in each, then cover with remaining batter.
07 - Bake for 10 to 12 minutes until donuts spring back when lightly touched and edges are golden brown.
08 - Let donuts cool in the pan for 5 minutes, then carefully transfer to a wire rack to cool completely.
09 - Whisk powdered sugar, milk, and vanilla extract in a bowl until smooth and reaches desired consistency.
10 - Dip tops of cooled donuts into the glaze. Immediately sprinkle with purple, green, and gold sugars in alternating sections to create King Cake stripes. Allow glaze to set before serving.