Preheat the oven to 350 degrees. Prepare an 8x8 baking pan by greasing with nonstick cooking spray.
In a medium bowl, whisk together the flour, baking powder, and salt.
In a separate bowl, beat together the butter, cream cheese, and sugar until creamy and well combined.
Add the egg, lemon juice, lemon zest, and lemon extract. Beat until combined.
Add the flour mixture and beat until smooth. Spread the batter into the prepared baking dish and bake for 25 to 30 minutes, until a toothpick inserted in the center comes out clean.
To make the frosting, beat together the butter, milk, lemon juice, lemon extract and powdered sugar until smooth and creamy. Add food coloring to tint icing if desired.
Allow the cookie bars to cool completely, then spread the frosting over the top of the cookie bars. Cut into 9 equal pieces before serving.
Notes
It is best to use fresh lemon juice for this recipe.