Prepare the baking equipment. Preheat the oven to 350°F (180°C). Cover a baking sheet with parchment paper or a Silpat silicone baking mat to help prevent sticking.

Combine the wet ingredients. Place the room-temperature softened butter and sweetened condensed milk in a large mixing bowl. Using either a hand mixer or stand mixer, beat the ingredients together until smooth and creamy.

Add the remaining flavoring ingredients. Mix in the vanilla extract, salt, and confectioners’ sugar. Continue beating until the mixture is evenly blended.

Incorporate the flour. Add the all-purpose flour approximately 1 cup at a time, mixing until each addition is incorporated before adding more. Avoid excessive mixing, as overworking the dough may result in firmer cookies.

Fold in the sprinkles. Once all of the flour has been incorporated, gently mix the rainbow sprinkles into the dough just until they are distributed throughout.

Shape the cookies. Portion the dough with a cookie scoop and roll or shape it into balls. Arrange the portions evenly across the prepared baking sheet. If desired, gently press each dough ball with your fingertips or a spatula to flatten it slightly.

Bake. Transfer the baking sheet to the preheated oven and bake for 8–10 minutes, until the cookie edges develop a light golden color.

Cool completely. Remove the cookies from the oven and transfer them to a wire cooling rack. Allow them to cool completely before decorating.

Finish with frosting. Once cooled, drizzle the cookies with vanilla frosting for an additional sweet and decorative finish. Allow the frosting to set before serving.