In a large bowl, cream together softened butter and granulated sugar until light and fluffy.

Beat in the egg and vanilla extract until well combined.

In a separate bowl, whisk together flour, baking soda, and salt.

Gradually add the dry ingredients to the wet ingredients, mixing until just combined. Do not overmix.

Cover the dough and refrigerate for at least 30 minutes.

Preheat oven to 375°F (190°C). Line baking sheets with parchment paper.

Roll the dough into 1-inch balls and place them about 2 inches apart on the prepared baking sheets.

Bake for 9-11 minutes, or until the edges are lightly golden. The centers should still look slightly underbaked.

Let the cookies cool on the baking sheets for a few minutes before transferring them to a wire rack to cool completely.

In a medium bowl, beat together softened butter and powdered sugar until smooth.

Add milk and vanilla extract and beat until the frosting is creamy and spreadable. Add more milk a teaspoon at a time if needed.

Once the cookies have cooled completely, spread a generous amount of frosting on top of each cookie.

Immediately sprinkle with colorful sprinkles before the frosting sets.