Put one of your oven racks in the middle of the oven. Preheat the oven to 350 F.
Line the bottoms of two 9” cake pans with parchment paper.
Grease the sides of the cake pans with butter or pan spray. (I prefer butter but either works.)
Put the cake mix in a large mixing bowl.
Add the milk, vanilla, eggs and the egg white (you do NOT need to beat the egg white – just toss it into the bowl).
Use a hand mixer and mix on low speed for 30 seconds.
Add the melted butter and use a hand mixer to beat for two minutes with a timer. (If beating by hand beat vigorously for two minutes with a timer!) Not beating the cake batter well can cause the cake to be dense and not rise well.
Divide the batter equally between the pans. Bake on the middle rack of your oven for 23-28 minutes. The cake is done when is starts to pull away a little bit from the sides and a toothpick or cake tester inserted comes out with moist crumbs but no batter.
After the cake is baked place the pans on a rack to cool until they are room temperature. (If you are in a hurry you can put the cakes in the refrigerator after the pans are cool enough to handle without oven mitts.
While the cake is cooling prepare the berries.
Wash and drain well.
Put half the blueberries, and half the raspberries into a measuring cup. Dice up strawberries and add them to the cup until you reach 3 cups total.
Set aside the remaining berries to decorate the cake.