In a large bowl or stand mixer, cream together butter and sugar. Add egg and vanilla and mix until combined. In a medium bowl, whisk flour, salt, baking powder, baking soda, and cornstarch. Gradually add dry ingredients to wet. Once well combined, cover dough and refrigerate at least 30 minutes.

Spread chilled dough on a well greased or lined 12-inch pizza pan. Bake until edges are lightly browned, 18-20 minutes.

In a medium bowl or stand mixer, beat cream cheese and butter together until smooth. Add remaining ingredents except fruit, and beat for about 3 minutes. Spread thickly over the cooled cookie crust. Decorate with fruit as desired.