Spray pan with non-stick spray or use parchment paper. Bake brownies in a 9x13 pan according to package directions. Cool completely.
Cut pan of brownies in half or quarters. (Whichever is easier for you to be able to remove the entire piece out!) Remove each piece and place on a cutting board. Use a medium flower cookie cutter to cut shapes into the brownies. (The flower cookie cutter I used measured 2.5" across) I was able to easily cut 14 flowers and had scraps left over for snacking.
Scoop frosting out into a bowl. You can use any variety of Pillsbury™ frosting, but we love the Vanilla Marshmallow. Beat the frosting about a minute so that it's smooth. Divide frosting in half. Put half into a pastry bag. Combine the other half with 1/2 cup finely diced strawberries.
Take a brownie flower cut-out and put a spoonful of frosting with strawberries on top. Stack with another brownie flower cut-out. Top with a piping of buttercream, sprinkles and a slice of strawberry.
Repeat with remaining brownies and frosting. Refrigerate leftovers.