I had a few cakes in mind so I decided to merge a few this is what I came up with, the use of extra eggs, milk instead of water & butter instead of oil is supposed to make it like a "bakery cake" it was very good but not sure I agree with that never the less everyone loved this cake and it is a keeper. I would imagine any flavor of cake and berry would work. If you freeze some of your cake freeze it with out the plastic wrap for a few hours first so it does not stick to the icing.
1Tips: these amounts are based on the cake box ingredients what your doing is replacing the oil with butter and double it. your adding 1 or 2 extra eggs and replacing the water with equal amounts of milk. so you can use any box mix or flavor you want just adjust to these amounts and mix according to the package. Also the original called for 2 1/2 cups of strawberries and 2 1/2 Tablespoon of cornstarch, my berries did not have much flavor and did need the full amount of sugar you can adjust next time I will add some strawberry preserves to the mixture, I did not included the 1 Tablespoon of espresso that I added in the ingredient list I did not feel it added much flavor ill try more next time.
2For the cake set oven to 350, grease and or line your pan or pans I used one 9 X 13 add to bowl cake mix, milk, melted cool butter mix lightly then add eggs I did mine one at a time mix well after each. use the cook times according to box directions test with tooth pick be-careful not to over bake nothing worse then a dry cake.
3For the filling, In a sauce pan medium heat ( I used my frying pan) add strawberries, sugar & cornstarch cook till thickened stir often crushing berries as they cook after all they are going between the cake layers. Remove from heat let cool completely before using. I did not use all mine so I froze the remaining cooked berries. I assume you can use any berry that you prefer.
4For the Icing, in a bowl add room temperature whip the butter till fluffy add vanilla mix well then cream in the cream cheese, slowly add the powdered sugar. if your not going to ice the cake store the icing and filling in the fridge like I did, bring back to room temp when ready to ice the cake.
5I wanted to expand on the icing for a minute. Many when making this sort of icing and butter cream icing find sometimes it may have a bit of a gritty feel. In my research I have found you dont want to over whip the cheese as it can separate which will split it or leave the grit feel it seems logically the sugar needs to have some liquid to melt the sugar and cornstarch.
Many do not recommend using table sugar finely ground as the starch is to prevent caking, you can see in my recipes I have made mine and had no issues. Saying that I would suggest using a splash of milk to aid in the spreading factor and melting the sugar to avoid the grit ALWAYS use room temperature butter and cheese.
6Back to the Cake, when cooled considering we are a family of two I cut my cake in half put the filling in and iced it I then froze half for a later time so far it seems to have done well in the freezer, ill update you when I eat it again. So for both sides slice your cake in half spread the cool strawberry mixture over the cake top with the other half. I put mine in the refrigerator at this point to let the cake and berries firm up then I spread the frosting back in the fridge.
7UPDATE: this cake actually tasted better after it was frozen and it seemed moister or both myself and my husband imagined it. Also the icing mine did have a bit of a grit feel I spoke about but after freezing it was gone yet another benefit to freezing half this cake.