In a large bowl or stand mixer, cream together butter and granulated sugar. Mix in egg, egg yolk and vanilla. When mixture is barely combined, add salt, baking powder and flour. Mix until no streaks remain and batter is lump free.
Cover and chill for at least one hour.
Preheat oven to 350°F.
Line a baking sheet with parchment paper or a silicone baking mat. Using a medium cookie scoop, scoop dough, shape into smooth balls and gently flatten slightly.
Bake for 10-11 minutes. Allow cookies to rest on baking sheet for 1 minute prior to transferring to a wire rack to cool completely.
When cookies have completely cooled, prepare frosting. In a medium bowl, mix together powdered sugar and butter until evenly dispersed. Add vanilla and 1 Tbsp milk or water at a time until desired icing consistency is reached.
Frost cookies and immediately top off with sprinkles. Allow frosting to set or enjoy immediately. One frosting has set completely, store cookies in an airtight container.