Bake the brownies in a 9x13 pan according to the package directions. Bake until the center is set and a toothpick comes out with moist crumbs, then remove and cool completely at room temperature for at least 1 hour (visual cue: frosting-ready surface with no steam).

Beat cream cheese, butter, powdered sugar, vanilla, and milk together until smooth and spreadable. Use a mixer until the frosting looks glossy and thick enough to hold stripes on top.

Spread the cream cheese frosting in an even layer over the cooled brownies. Cover the entire surface so the design has a smooth white base (visual cue: frosting reaches the edges of the pan evenly).

In the upper left corner, arrange a rectangle of blueberries tightly packed to form the canton. Press lightly so the berries stay flat and touch for clean edges.

Create red stripes across the rest of the brownies using rows of sliced strawberries laid flat. Align the rows horizontally so they read as stripes over the white frosting.

Leave alternating gaps between strawberry rows as the white stripe showing through the frosting. Make the gaps consistent in width so the pattern is clearly defined.

Refrigerate the brownies for 30 minutes to set the frosting, then cut into squares and serve. For best slices, chill until the frosting feels firm (visual cue: frosting doesn’t smear when the knife moves through).