01 - Preheat oven to 350°F. Grease a standard donut pan thoroughly with cooking spray or butter.
02 - In a large bowl, whisk together flour, granulated sugar, baking powder, cinnamon, salt, and lemon zest until well combined.
03 - In a separate bowl, whisk eggs, milk, melted butter, and vanilla extract until smooth and fully incorporated.
04 - Pour wet ingredients into dry ingredients. Fold gently until just combined—batter should remain slightly lumpy. Avoid overmixing to prevent dense donuts.
05 - Stir together melted butter, brown sugar, and cinnamon in a small bowl until sugar begins to dissolve into a paste-like consistency.
06 - Spoon half the donut batter into prepared donut pan molds, filling each about one-third full. Add a small swirl of cinnamon sugar filling to each mold, then cover with remaining batter to fill molds three-quarters full.
07 - Bake for 10 to 12 minutes until donuts spring back when lightly touched and edges are golden brown. Cool in pan for 5 minutes before transferring to wire rack.
08 - Whisk powdered sugar, milk, and vanilla extract in a small bowl until completely smooth and free of lumps. Glaze should coat the back of a spoon.
09 - Dip tops of cooled donuts into glaze, allowing excess to drip off. Immediately decorate with purple, green, and gold sprinkles while glaze remains wet. Let set for at least 10 minutes before serving.