01 - Preheat oven to 350°F. Line a 12-cup muffin tin with paper liners.
02 - Whisk together flour, baking powder, and salt in a medium bowl. Set aside.
03 - Beat butter and granulated sugar in a large bowl until light and fluffy. Add eggs one at a time, mixing well after each addition. Stir in vanilla extract.
04 - Gradually add flour mixture to butter mixture, alternating with milk, beginning and ending with flour. Mix until just combined. Gently fold in ground cinnamon.
05 - Combine brown sugar and cinnamon in a small bowl for the swirl filling.
06 - Fill cupcake liners halfway with batter. Sprinkle about 1/2 tsp cinnamon sugar swirl over each, then top with remaining batter. Gently swirl with a toothpick for a marbled effect.
07 - Bake for 18-20 minutes, or until a toothpick inserted into the center comes out clean. Cool in pan for 5 minutes, then transfer to a wire rack to cool completely.
08 - Whisk together powdered sugar, milk, and vanilla extract until smooth and pourable.
09 - Spoon or drizzle icing over cooled cupcakes. Immediately sprinkle with colored sugars in alternating stripes of purple, green, and gold. Allow icing to set before serving.