Preheat the Oven
Preheat your oven to 350°F (175°C) and line a 9x13-inch baking pan with parchment paper or grease it well.
Make the Cookie Bar Dough
In a mixing bowl, cream the butter and sugar with an electric mixer on medium speed until light and fluffy, about 2-3 minutes.
Beat in the eggs one at a time, then add the vanilla extract and mix until combined.
In a separate bowl, whisk together the flour, baking powder, and salt. Gradually add the dry ingredients to the wet ingredients, mixing until just combined.
Bake the Bars
Spread the dough evenly into the prepared pan using a spatula or your hands. Bake for 18-20 minutes, or until the edges are lightly golden and the center is set. Allow the bars to cool completely before frosting.
Make the Frosting
In a clean mixing bowl, beat the butter until creamy.
Gradually add the powdered sugar, 1 cup at a time, mixing on low speed.
Add the milk or heavy cream, vanilla extract, and food coloring (if using). Increase the speed to high and beat until the frosting is light and fluffy, about 2-3 minutes.
Frost and Decorate
Spread the frosting evenly over the cooled cookie bars. Add sprinkles or decorations as desired.
Slice and Serve
Cut the bars into squares or rectangles and serve.