Prep: Preheat the oven to 375 degrees F. Then line a 9 x 13 baking dish with parchment paper or spray with non-stick cooking spray, set aside.
Wash the raspberries and ensure all stems/leaves are removed. Zest the lemon for the required measurements for the batter and frosting and then squeeze for the lemon juice as well.
Cookie dough: In a medium sized bowl, combine the flour, baking powder, and salt, set it aside. In a separate large mixing bowl, cream the butter and sugar for about 2-3 minutes until it is light and fluffy. Add the egg, sour cream, lemon juice, lemon zest, and vanilla extract. Mix until combined.
Slowly add in the flour mixture, while stirring, until everything is incorporated. Fold in the fresh raspberries and mix once more.
Bake: Press the batter into the prepared baking dish and spread it out evenly with your hands or a rubber spatula. Bake the sugar cookie bars for 15-17 minutes or until the edges have just started to turn golden. Be sure to allow the cookie bars to cool completely before adding the frosting.
Frosting: In a large bowl, combine the butter, lemon juice zest, milk, vanilla extract, powder sugar, food coloring (if desired) and use an electric mixer to combine ingredients well. Beat the ingredients for 2-3 minutes until the frosting is smooth and creamy. You can add more powdered sugar or milk if needed, until you reach the desired consistency.
Assemble: Spread the frosting evenly over the cooled cookie bar, then cut into squares and serve with additional raspberries on top. Enjoy!