01 - Preheat oven to 350°F. Line a 12-cup muffin tin with paper liners.
02 - In a medium bowl, whisk together flour, baking powder, and salt. Set aside.
03 - In a large bowl, beat butter and granulated sugar until light and fluffy. Add eggs one at a time, mixing well after each addition. Stir in vanilla extract.
04 - Gradually add the flour mixture to the butter mixture, alternating with milk, beginning and ending with flour. Mix until just combined.
05 - Gently fold in ground cinnamon.
06 - In a small bowl, combine brown sugar and cinnamon for the swirl.
07 - Fill cupcake liners halfway with batter. Sprinkle about 1/2 teaspoon cinnamon sugar swirl over each, then top with remaining batter. Gently swirl with a toothpick for a marbled effect.
08 - 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.
09 - For the icing, whisk together powdered sugar, milk, and vanilla extract until smooth and pourable.
10 - Spoon or drizzle icing over cooled cupcakes. Immediately sprinkle with colored sugars in alternating stripes (purple, green, and gold/yellow). Allow icing to set before serving.