3/4 cup unsalted butter, room temperature, divided
3/4 cup granulated sugar
1 egg, room temperature
2 teaspoons vanilla extract, divided
1 and 1/2 cups all-purpose flour
1 and 1/2 teaspoons cornstarch
1 teaspoon cornstarch
1 teaspoon baking powder
1/2 teaspoon baking soda
1/4 teaspoon kosher salt
1 package (8 ounces) cream cheese, room temperature
1 and 1/2 cups confectioners' sugar
2-3 tablespoons heavy cream
1 cup blueberries
2 cups strawberries, hulled, and halved
1 can whipped cream
Beat 1/2 cup butter with an electric mixer one medium speed until creamy, about 1 minute. Add granulated sugar, and beat on medium speed until fluffy. Add egg and 1 teaspoon vanilla, and beat until smooth, scraping sides of bowl as necessary. In a small bowl, whisk together flour, cornstarch, baking powder, baking soda, and salt. Gradually add flour mixture to butter mixture while mixer is on low speed. Beat until completely combined, about 1 minute. Transfer dough to a bowl, cover tightly with plastic wrap and chill 30 minutes.
Preheat oven to 350 degrees. Lightly coat a 13-by-9-inch baking sheet with cooking spray. Press dough evenly into baking sheet, forming a slight edge on all sides. Bake until edges are very lightly browned, 18-20 minutes. Cool completely.
Beat cream cheese and remaining 1/4 cup butter on medium speed until smooth, about 2 minutes. Add confectioners' sugar and 2 tablespoons cream; beat 2 minutes. Add remaining 1 teaspoon vanilla and tablespoon of cream. Beat 1 minute. Spread in a thick layer over cookie.
Arrange blueberries in a square in the top left corner of the cookie. Arrange strawberry halves in parallel horizontal lines, leaving about an inch between each row. Then squirt whipped cream stars in between each strawberry row. Slice and serve.