01 - Preheat oven to 350°F. Grease a standard donut pan thoroughly with cooking spray or butter.
02 - In a large bowl, whisk together all-purpose flour, granulated sugar, baking powder, cinnamon, salt, and lemon zest until well combined.
03 - In a separate bowl, whisk eggs, whole milk, melted butter, and vanilla extract until smooth and fully incorporated.
04 - Pour wet ingredients into dry ingredients. Mix gently until just combined, being careful not to overmix the batter.
05 - Stir together melted butter, light brown sugar, and ground cinnamon in a small bowl until a uniform paste forms.
06 - Spoon half the donut batter into the prepared pan molds. Add a small swirl of cinnamon sugar filling to each, then cover with remaining batter.
07 - Bake for 10 to 12 minutes until donuts spring back when lightly touched and edges are golden. Cool in pan for 5 minutes, then transfer to wire rack.
08 - Whisk powdered sugar, whole milk, and vanilla extract in a small bowl until completely smooth and drizzly.
09 - Dip tops of cooled donuts into glaze, allowing excess to drip off. Immediately decorate with purple, green, and gold sprinkles while glaze is wet.
10 - Allow glaze to set for at least 10 minutes at room temperature before serving. Best enjoyed the same day.