01 - Whisk together flour, baking powder, and salt in a medium bowl. Set aside for later use.
02 - Beat butter and granulated sugar in a large bowl until light and fluffy, approximately 2-3 minutes.
03 - Add the egg and vanilla extract to the butter mixture, mixing until fully incorporated.
04 - Gradually add the dry ingredients to the wet mixture, mixing just until the dough comes together. Avoid overmixing.
05 - Divide dough in half, shape into flat discs, wrap tightly in plastic wrap, and refrigerate for 30 minutes.
06 - Preheat oven to 350°F. Line baking sheets with parchment paper and set aside.
07 - On a lightly floured surface, roll out one dough disc at a time to 1/4-inch thickness. Cut out shapes using star or round cookie cutters.
08 - Place cut cookies on prepared baking sheets, leaving 1 inch of space between each cookie.
09 - Bake for 9-11 minutes until edges are lightly golden. Allow cookies to cool on the baking sheet for 5 minutes, then transfer to a wire rack to cool completely.
10 - In a bowl, combine powdered sugar, 2 tablespoons milk, corn syrup, and vanilla extract. Mix until smooth. Add additional milk if needed to achieve thick but pourable consistency.
11 - Divide icing between two bowls. Tint one bowl with deep blue coloring, the other with black. Gently swirl the two colors together in a shallow dish to create a marbled pattern.
12 - Dip cooled cookies face-down into the marbled icing. Immediately sprinkle with edible star decorations while icing is still wet. Allow icing to set for at least 30 minutes before serving.