01 - Preheat oven to 350°F. Grease a 12-cavity donut pan with cooking spray or butter.
02 - In a large bowl, whisk together flour, granulated sugar, baking powder, baking soda, salt, cinnamon, and nutmeg until evenly combined.
03 - In a medium bowl, whisk buttermilk, eggs, melted butter, vanilla extract, and lemon zest until fully incorporated.
04 - Pour wet ingredients into dry ingredients and fold gently until just combined. Do not overmix to maintain tender crumb structure.
05 - In a small bowl, beat cream cheese, powdered sugar, and vanilla extract until smooth and creamy. Set aside if using.
06 - Transfer batter to piping bag. For filled donuts, pipe a thin layer of batter into each cavity, add small amount of cream cheese mixture, then top with additional batter. For unfilled donuts, fill each cavity approximately two-thirds full with batter.
07 - Bake for 10 to 12 minutes until donuts spring back when lightly pressed. Allow to cool in pan for 5 minutes, then transfer to wire rack for complete cooling.
08 - Whisk powdered sugar, milk, and vanilla extract until smooth and pourable consistency is achieved.
09 - Dip cooled donuts into glaze, then immediately sprinkle with purple, green, and gold sanding sugars in sections to create King Cake appearance. Allow 5 minutes for glaze to set before serving.