Preheat your oven to 350°F (175°C) and grease your 9x13 inch baking pan, or line it with parchment paper for easier removal.

In a large mixing bowl, beat the softened butter and granulated sugar together until light and fluffy using an electric mixer or whisk.

Add the eggs one at a time, mixing well after each addition. Stir in the vanilla extract.

In a separate bowl, whisk together the flour, baking powder, baking soda, and salt until well combined.

Gradually add the dry ingredients to the butter mixture, stirring until just combined. Be careful not to overmix; the dough should be soft and slightly sticky.

Spread the dough evenly into the prepared baking pan. Use a rubber spatula to smooth the top.

Bake in the preheated oven for 20 to 25 minutes, or until the edges are lightly golden and a toothpick inserted in the center comes out clean.

Allow the sugar cookie bars to cool completely in the baking pan on a wire rack.

For the frosting, beat the softened butter in a mixing bowl until creamy. Gradually add the powdered sugar while mixing at low speed, then add the milk and vanilla extract. Beat until smooth and fluffy.

Once the bars have cooled, spread the frosting evenly over the top and sprinkle with sprinkles if desired.

Cut into squares and serve.