Step 1: Preheat your oven to 350°F (175°C). Grease your baking pan with non-stick spray or line it with parchment paper.
Step 2: In a medium bowl, whisk together the all-purpose flour, baking powder, and salt. Set aside.
Step 3: In a large bowl, cream 3/4 cup of softened unsalted butter and granulated sugar together until light and fluffy, about 2-3 minutes.
Step 4: Add the two large eggs, one at a time, mixing well after each addition. Stir in the vanilla extract until well combined.
Step 5: Gradually add the dry mixture to the wet ingredients, mixing on low speed until just combined. Be careful not to over-mix.
Step 6: Spread the cookie dough evenly into the prepared baking pan. Bake for 20-25 minutes or until the edges are lightly golden and a toothpick comes out clean. Let cool completely.
Step 7: In a medium bowl, beat together 1/2 cup of softened unsalted butter and 2 cups of sifted powdered sugar until creamy. Add milk and vanilla extract; mix until smooth.
Step 8: Once the bars are completely cool, spread the frosting generously over the top. Add food dye and sprinkles if desired.