Step 1: Preheat your oven to 350°F (175°C). Grease and flour a 9×13 inch baking pan, or line it with parchment paper leaving an overhang on the sides for easy removal.
Step 2: In a large bowl, cream together the softened unsalted butter and granulated sugar using an electric mixer until light and fluffy, about 2-3 minutes.
Step 3: Beat in the eggs one at a time, ensuring each is fully incorporated before adding the next. Stir in 2 teaspoons of vanilla extract. Scrape down the sides of the bowl.
Step 4: In a separate medium bowl, whisk together the all-purpose flour, baking powder, and salt. Gradually add the dry ingredients to the wet ingredients, mixing on low speed until just combined. Do not overmix.
Step 5: Press the cookie dough evenly into the prepared 9×13 inch baking pan. Bake for 18-22 minutes, or until the edges are lightly golden and a toothpick inserted into the center comes out with moist crumbs. Remove from the oven and let cool completely in the pan on a wire rack.
Step 6: While the bars cool, prepare the frosting. In a large bowl, beat the softened unsalted butter until creamy and smooth, about 2 minutes.
Step 7: Gradually add the 4 cups of sifted powdered sugar, about 1 cup at a time, beating well after each addition until fully incorporated.
Step 8: Add the 1/2 cup heavy cream (or milk) and 1 teaspoon of vanilla extract. Beat on medium-high speed for another 2-3 minutes, until the frosting is light, fluffy, and smooth.
Step 9: Once the cookie bars are completely cool, spread the vanilla buttercream frosting evenly over the top. If desired, sprinkle with colorful sprinkles immediately after frosting.
Step 10: Cut the frosted cookie bars into squares and serve!