Preheat your oven to 375 degrees.
In a medium sized bowl, combine your flour, baking powder, salt and lemon zest. Set bowl aside.
In a large mixing bowl, cream together your butter and sugar for about 3 minutes until light and fluffy.
Add in your egg, sour cream, lemon juice, and vanilla and mix till combined.
Slowly add in your flour mixture while stirring until everything is combined.
Press the batter into a greased 9x13 dish and spread it out evenly with your hands or a rubber spatula.
Bake your sugar cookie bars for 15-17 minutes or until the edges have just started to turn golden.
Allow cookie bars to cool completely.
Mix all your ingredients together for your frosting until creamy. Add more powdered sugar or milk if needed for the texture.
Spread your frosting evenly over your cooled cookie bars.
Cut into squares and serve.