Preheat oven to 350°F (175°C). Grease and flour a 9x13 inch baking pan.

In a large bowl, cream together softened butter and granulated sugar until light and fluffy.

Beat in the egg and vanilla extract until well 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. Do not overmix.

Spread the dough evenly into the prepared baking pan.

Bake for 20-25 minutes, or until the edges are golden brown and a toothpick inserted into the center comes out clean.

Let the bars cool completely in the pan on a wire rack.

While the bars are cooling, prepare the frosting. In a medium bowl, beat softened butter until creamy.

Gradually add the powdered sugar, alternating with the milk, until smooth and creamy.

Stir in the vanilla extract.

Once the cookie bars have completely cooled, spread the frosting evenly over the top.

Cut into squares and serve.