Design Eat Repeat

2 Be sure to read through the blog post above for important tips & tricks (and step-by-step photos!).
Making The Cake
0 In a large mixing bowl, stir together boxed dry yellow cake mix ([adjustable]15.25[/adjustable] ounces), melted unsalted butter ([adjustable]½[/adjustable] cup), milk ([adjustable]¾[/adjustable] cup), vanilla extract ([adjustable]1[/adjustable] teaspoon), eggs ([adjustable]3[/adjustable] large), and extra egg yolk ([adjustable]1 [/adjustable]large). Pour in greased and parchment-lined 9x13 light-colored metal baking pan, spread until smooth, then tap pan on counter about 20 times to remove the air bubbles.
1 Bake at 350°F for 20-22 minutes or until toothpick comes out clean. Allow cake to cool for 10 minutes, then flip over onto a wire rack or piece of parchment paper. If your cake has a bit of a dome, press the pan on top as "weight" to help it flatten while it continues cooling.  
3 Once cooled, freeze cake for 30-60 minutes. This will help the cake firm up so that you can cut it into circles without it crumbling. 
Make Buttercream
6 Using an electric mixer (if using a Kitchenaid stand mixer, use the whisk attachment), cream unsalted butter ([adjustable]¾[/adjustable] cup) and salt ([adjustable]¼[/adjustable] teaspoon) on high speed for 2 minutes until soft and creamy.
7 Slowly mix in powdered sugar 1/2 cup at a time ([adjustable]4 ½[/adjustable] cups total), add vanilla extract ([adjustable]1[/adjustable] tablespoon), then beat for 5-8 minutes at medium-high speed. It’s important that you beat this for at least 5 minutes for it to be fluffy & creamy. If your frosting is thick, you can add additional teaspoons of milk ([adjustable]3-4[/adjustable] teaspoons) until desired consistency.
8 Put frosting in a piping bag with large round tip or ziploc bag with the large corner cut off, then set aside for next step.
Cut Cake & Pipe Filling
10 Remove cake from the freezer then immediately cut it lengthwise into two layers with a serrated knife.
12 On the bottom half, pipe half of the frosting and spread. Place the other half of the cake on top and gently press down.
Topping
19 Pipe the rest of the frosting on top of the cake, then use a spatula to spread it into an even layer.
20 Finish it off by melting chocolate chips ([adjustable]¼[/adjustable] cup) and shortening ([adjustable]1[/adjustable] teaspoon, optional) in the microwave at 50% power in 30-second increments, then put it in a ziploc bag with a tiny corner cut off to pipe on “zebra” stripes on top.