Preheat the oven to 350°F (175°C) and grease a baking pan.

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

Add the egg and vanilla extract, mixing until well combined.

In another bowl, whisk together the flour, baking powder, and salt.

Gradually add the dry ingredients to the wet ingredients, mixing until just combined.

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 inserted in the center comes out clean.

Allow to cool completely in the pan.

In a mixing bowl, combine the powdered sugar, softened butter, milk, and vanilla extract.

Beat until smooth and creamy.

Spread the frosting over the cooled cookie bars.

Cut into squares and serve.