Preheat your oven to 375 degrees.
In a medium sized bowl, combine your flour, baking powder and salt. Set 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 and vanilla and mix till combined.
Slowly add in your flour mixture while stirring until everything is combined. (The dough will be crumbly)
Press the batter into a greased 9×13 baking dish and spread it out evenly with your hands or a rubber spatula.
Bake for 14-16 minutes or until edges just barely start to turn golden.
Allow to cool completely.
Meanwhile, puree your strawberries. Heat puree over medium low heat in a small sauce pan for about 15 minutes, or however long it takes the puree to condense half way. (So you'll want to end up with 1/4 cup)
Place in your freezer and allow to cool for a few minutes.
Mix together your strawberry puree and your butter.
Add in your powdered sugar, milk and vanilla extract and mix everything together till smooth. (Add a little more milk or a little more powdered sugar if needed till you get a spreadable consistency).
Spread frosting evenly over cooled sugar cookie bars.
Cut into squares and serve.