Preheat the oven to 350°F. Line 3 large baking sheets with parchment paper and set aside.

Sift together the all-purpose flour, baking powder, baking soda, and salt in a medium bowl, set aside.

In a large bowl cream together, using a handheld mixer on medium speed, the unsalted butter and granulated sugar for 2-3 minutes or until light and fluffy.

Add the eggs and egg yolk, one at a time being sure to incorporate well between each addition, sour cream, and vanilla to the bowl. Mix on medium speed just until combined.

Add the dry ingredients to the bowl of wet ingredients and mix on low just until fully combined. The batter will be thick.

Scoop out 12 level scoops (using a 1.5 tablespoon sized cookie scoop) onto each of the prepared baking sheets being sure that the dough balls are spaced about 2 inches apart.

Bake for 10-11 minutes or just until the edges are very lightly browned and the centers are set. Allow the cookies to cool on the baking sheet for 10 minutes before transferring to a wire rack to cool completely. If needed, rotate the baking sheets half way through to ensure even baking.

To the bowl of a stand mixer fitted with the paddle attachment, beat the butter on medium-high speed for 2-3 minutes or until light and fluffy.

Add the powdered sugar, vanilla extract, salt, and 3 tablespoons of heavy cream. Beat on low speed until all the powdered sugar is incorporated then increase the speed to medium and beat until light and fluffy. The frosting should be thick enough to hold its shape when spread onto the cooled cookies. If too thick, beat in the additional 1 tablespoon heavy cream.

Add gel food coloring (starting with 2-3 drops and adding more until desired color achieved) mixing until no streaks remain and the frosting is uniformly colored.

Once the cookies are completely cooled, spread the frosting evenly over the tops of each cookie and garnish with rainbow-colored nonpareil sprinkles if desired.