1. Prepare the Crust:

1. In a large bowl, cream together the softened butter and granulated sugar until light and fluffy. Beat in the egg and vanilla extract.
2. In a separate bowl, whisk together the flour, baking powder, and salt. Gradually add the dry ingredients to the wet ingredients, mixing until a smooth dough forms. Divide dough in half, shape into discs, wrap in plastic, and chill for at least 1 hour.
3. Preheat oven to 375°F (190°C). On a floured surface, roll out one disc of dough to 1/4-inch thickness. Use snowflake-shaped cookie cutters to cut out shapes and place them on parchment-lined baking sheets. Bake for 7-9 minutes, or until the edges are just lightly golden. Let cool completely on a wire rack.
4. For the icing, whisk together powdered sugar and milk until smooth. Add more milk for a thinner consistency if desired. Add a few drops of blue food coloring if using. Decorate the cooled cookies with the icing and allow it to set completely before serving.