This is one recipe where I recommend using the brand name items shown for the cake mix and the powdered sugar. I've tried off brands, and the frosting will turn out lumpy every time if you don't use the brand name. It's worth the extra dollar for the best frosting ever.
Preheating your oven to the temperature suggested for a rectangular pan on the back of your cake mix box. Put your mix, water, oil, and eggs into a medium bowl.
Mix it up with a whisk by hand until everything is incorporated and all large lumps are gone. Don't follow the "beat it with a hand mixer for 2 minutes" instruction on the back of the box. This makes a tough, dry cake. We want a moist, fluffy cake.
Now grab your baking pan and put some crisco on a paper towel. Rub it all over the bottom and sides of the pan. Then you're going to put some cocoa powder (for a chocolate cake) or flour (for any lighter cake) on the bottom of your pan and start rolling and tapping it around until the pan is completely coated. This is my magic trick for making sure that cakes do not stick to the pan and come out pretty every time!
While you let your cake cool completely, let's make our frosting.
I recommend using a stand mixer with the paddle attachment if you have one. If you don't, a hand mixer will work. However, it will take longer at each step to make the frosting smooth.
Put your crisco stick and your two soft butter sticks in the mixer, and mix on medium until nice and smooth. Then add in your vanilla and mix again.
Now you're going to start mixing in your powdered sugar. I use a full 2 lb bag, and pour in about 1 cup at a time. Mix well until all lumps are gone between each addition of powdered sugar. Take your time with this. The slower you go and the more you mix, the smoother and creamier your frosting will be. Make sure to scrape the sides of the bowl regularly.
It should look very smooth at the beginning when adding the sugar. Now when all your sugar is incorporated, your frosting will be crazy thick. It should look like a scoop of vanilla ice cream. Start adding milk, a tablespoon at a time, and mix well. You should use 4 tablespoons total, and when you're done you'll have a medium thick, pipeable and spreadable frosting.
Grab a piping bag and a large star tip if you have one. If you don't you can use a quart size plastic bag and cut the tip off. You'll just have cute round piping rather than star piping.
Start placing your blueberries in the upper left corner of your cake. I did a section that was 5 blueberries by 7 blueberries. Then I piped little stars in between the berries. You can also pipe little dots!
When your stars section is done, get your raspberries. Place your raspberries in a straight line across the top of the cake. I alternated mine with one pointing up and one pointing down to create a smooth line and get lots of berries on the cake. Then take your piping bag and pipe a line across the bottom of the berries.
You can decorate the tray with some extra berries if you'd like. Normally I don't recommend refrigerating cakes, but this one does best kept in the fridge to keep the berries fresh! It keeps for 2-4 days.