Place a rack in center of oven; preheat to 350°. Lightly spray a 13" x 9" baking dish with cooking spray.

In a large bowl, prepare cake batter according to package directions. Pour into prepared baking dish; smooth top.

Bake cake until golden brown and a tester inserted into the center comes out clean, 25 to 30 minutes. Let cool.

Meanwhile, in the large bowl of a stand mixer fitted with the whisk attachment (or using a handheld mixer), beat cream cheese, granulated sugar, and a pinch of salt on medium speed until light and fluffy, about 2 minutes.

Gradually add cream and vanilla, scraping down sides of bowl as needed, and beat until mixture is fluffy and holds its shape, about 1 minute.

Spread about half of frosting over surface of cake.

Transfer remaining frosting to piping bag fitted with small star tip. Pipe a border in top left hand corner to section off an area for the stars, if desired.

Fill section with tight rows of blueberries to cover frosting, filling in the gaps between each berry with a second layer of blueberries on top.

Pipe a small dollop on top of exposed blueberries to resemble stars. Dust with confectioners' sugar (if using).

Starting at top edge of cake, snugly arrange raspberries in 2 even rows. Repeat with remaining raspberries, creating 4 rows and leaving a blank space in between each row.

Pipe stripes, swirls, or any pattern you wish with remaining frosting in blank spaces.