Preheat the oven to 350 degrees F and grease a 9x13 inch cake pan and line it with parchment paper. Set aside.
In a medium bowl, whisk together the flour, baking powder and soda, salt, and xanthan (if using).
In the bowl of a stand mixer fitted with a paddle attachment, beat the butter until it's light and fluffy. Add the sugar and beat until fluffy. Add the eggs, one at a time, and beat until they are fully incorporated.
Add half the flour to the bowl, along with the milk and sour cream. Beat until mixed. Add the remaining flour and beat until fully incorporated. Scrape the batter into the prepared pan, place the cake on the middle of the center rack, and bake for about 30 minutes, or until a toothpick inserted in the middle of the cake comes out clean.
Let the cake cool in the pan for 15 minutes, then turn the cake out to a cooling rack to finish cooling completely.
Meanwhile make the frosting. Beat the butter until light in the bowl of a stand mixer fitted with a paddle attachment. Add the sifted powdered sugar and beat on low while slowly drizzling in the milk. The frosting should be light, fluffy and somewhat thick. If it's too stiff, add additional milk one tablespoon at at time. Add the vanilla extract and pinch of salt and beat to combine.
Once the cake has cooled, spread the entire cake evenly with a layer of buttercream frosting. Then, fit a piping bag with the French star tip and pipe star shapes all around the edge of the cake. At the top left of the cake, pipe a small rectangle of frosting stars where you will set the blueberries. This cake has 6 frosting stars across, in a total of 5 rows. Nestle the blueberries in between the frosting stars.
At the top left of the cake, pipe a small rectangle of frosting stars where you will set the blueberries. This cake has 6 frosting stars across, in a total of 5 rows. Nestle the blueberries in between the frosting stars.
Next, to the right of the blueberry stars, lay out a row of sliced strawberries just under the top edge of the cake. Pipe a decorative row of frosting stars under the sliced strawberries then lay out a row of upside down raspberries across the cake.
Repeat with a row of frosting stars and sliced strawberries. Repeat with a row of frosting stars and upside down raspberries. Continue in this pattern until the cake is finished. I ended up with three rows of sliced strawberries, interspersed with two rows of raspberries.
Finish the cake by piping a frosting stars around the bottom edge of the cake. Chill until ready to serve.
Though this cake is somewhat time consuming to decorate, it can be made up to a day in advance. The undecorated cake can be made a few weeks in advance, wrapped tightly and frozen.