Whisk softened butter until creamy (1-2 minutes). You can use an electric mixer if you prefer.

Add sugar, and whisk until light and airy (1-2 minutes).

Add vanilla and egg, and whisk until fully mixed in (30 seconds-1 minute).

In a separate bowl, mix together dry ingredients.

Gradually add dry ingredients to wet ingredients, stirring until fully combined. Start with a whisk, and switch to a spoon as the dough becomes thicker.

Press dough into greased 13x9 inch pan. Make sure dough is spread out evenly.

Bake 20-25 minutes at 350F. When the edges and/or top begin to lightly brown, the cake is ready. Be sure not to over bake, as cookie will become dried out. We want it to be soft.

Allow cake to cool completely. If you can't add the topping as soon as it cools, cover the pan tightly with plastic wrap until you are able to work on it (no more than one day later).

Using a fork, whip the cream cheese, milk, and powdered sugar until smooth (1-2 minutes). Use a spatula and knife to spread evenly over cookie cake. Add strawberries and blueberries.