Preheat the oven to 350°F (177°C). Line an 8-inch or 9-inch square pan with aluminum foil, ensuring enough overhang on the sides.

In a large bowl, cream the softened butter for about 1 minute on medium speed until smooth. Add the sugar and beat until fluffy and light in color. Beat in the egg, egg yolk, and vanilla, scraping down the sides as needed.

In a medium bowl, whisk together the flour, salt, baking powder, baking soda, and cornstarch. With the mixer on low speed, slowly add the dry ingredients to the wet ingredients in 3 parts. Gently fold in the sprinkles using a silicone spatula.

Press the cookie dough evenly into the prepared baking pan. Bake for 25-26 minutes until lightly browned on top. Allow the bars to cool completely on a wire rack before frosting.

For the frosting, beat the butter on medium speed until creamy, about 2 minutes. Add confectioners’ sugar, cream, and vanilla with the mixer on low. Increase to high speed and beat for 3 minutes. Adjust the consistency with more sugar or cream as needed and add a pinch of salt if too sweet.

Remove the cooled bars from the pan using the aluminum foil overhang. Frost the bars and decorate with extra sprinkles if desired. Cut into squares using a sharp knife wiped between cuts for neat edges. Store in an airtight container at room temperature for up to 2 days or in the refrigerator for up to 5.