Begin by combining the all-purpose flour, salt and boxed cake mix together. Sift them together to get rid of any lumps. (You can also use a whisk, that works just as well).
In a stand mixer, place the unsalted room temperature butter inside the bowl and by using the paddle mix the butter until smooth and creamy.
Then add the sugar, mix again until light and fluffy. Scrape down the sides of the bowl with a rubber spatula. Now add the oil from the cake mix recipe.
Slowly add in all the eggs from the pound cake and the cake mix (it should be around 7-8 eggs depending on the cake mix). Then add the vanilla extract.
Remember to scrape down the sides of the bowl every few eggs to make sure everything is getting incorporated. At this point the batter will look broken and weird and you are probably thinking something went wrong no that’s okay, I promise it will look better after the next step!
Now add the all-purpose flour, salt and the cake mix (dry ingredients) to the mixer, it will become thick but still creamy.
Next, slowly add the water (or milk is fine too) from the cake mix. Your goal at this time is just to mix the two recipes together to create a combined batter. Once you don’t see any visible dry ingredients turn off the mixer and you are done!
This is the point when you do your last scrape down the sides of the bowl including the bottom of the bowl (get in there). This batter is really forgiving and will be fine.
Pour or scoop the batter into the prepared pans either sprayed down and then lined with parchment paper or shortening rubbed and coated in flour. (I personally always line the bottom of the cake pans so when I flip the cake out of the pan it’s easier to handle with paper underneath).
Bake according to the cake mix directions (typically 20-25 minutes depending on the size of the cake pan), check the center of the cake with a toothpick and once it comes out clean then it’s done.
Let cool slightly so you can flip the cake out onto a cooling rack to finish cooling. This will prevent the cake from drying out.
Either wrapping plastic to freeze for the next time you want cake and can decorate it or buttercream and decorate it once it’s completely cooled. Even if you are decorating the same day as you baked the cake, still freeze the cake for 2 hours because it is so easy to frost a cold cake. Now you can start decorating!