Measure all of your ingredients before you begin making this recipe. Be sure to divide ingredients in separate containers such as (a) 1-1/8 cups milk with the vanilla extract for the yellow batter; (b) 1/3 cup milk for the chocolate batter, (c) 2 tablespoons milk for the yellow batter once divided. Also, divide the eggs (5 yolks, 1 yolk, and 6 whites) into separate bowls. You can actually set the egg whites in your mixing bowl until they reach room temperature.
Preheat oven to 350°F. Butter two 9-inch round cake pans and dust with flour. Line with parchment paper; butter the top of paper. Wrap the pans in moistened cloth (such as Wilton’s Cake Belts or paper towels wrapped in foil for level cakes with tender sides that are not brown. (See the above-linked article.)
In a separate large mixing bowl, whisk together all 6 egg whites and granulated sugar until foamy, about 2 minutes.
Whisk in 1-1/2 cups milk, the butter, oil, only 6 egg yolks, and vanilla until smooth. Add flour mixture; whisk until smooth.
In a separate large bowl, whisk together the remaining egg yolk, 1/3 cup milk, and the cocoa powder.
Add 2-1/4 cups of vanilla batter to cocoa mixture and whisk until smooth. Whisk 2 tablespoons milk into the remaining vanilla batter. If your batter is too thick feel free to stir in a little bit of additional milk to either.
Spoon 1/3 cup vanilla batter into the center of each prepared pan. Then, spoon 1/4 cup of the chocolate batter into the center of each vanilla circle. Repeat, always spooning batter into center of pan and occasionally tapping pan firmly on counter, until all batter is used (batters should appear as concentric rings). Watch the video at the above link.
Bake, rotating pans halfway through, until cakes are puffed slightly, a cake tester comes out clean, and cakes spring back when lightly touched, about 32 minutes.
Transfer pans to a wire rack to cool 15 minutes.
Remove the pans wrappers, loosen edges of cakes from sides, turn out onto an oiled rack to cool completely, 20 - 30 minutes, and remove and discard parchment.
In the bowl of your electric mixer fitted with the paddle attachment, set to medium speed, beat the butter and confectioners’ sugar until light and fluffy, about 5 minutes.
Whisk together the cocoa powder, espresso powder, salt, hot water, and vanilla until smooth.
Set the bowl of cocoa powder mixture over a small saucepan if simmering water (not touching the water) and add in the chopped chocolate/chocolate chips. Stir occasionally until it completely melts; set aside to cool for 5 minutes.
Remove the bowl from the saucepan; stir in the corn syrup until smooth.
Add 1/4 of this chocolate mixture at a time to the butter mixture, mixing on low speed until combined.
Spread about 1/4-inch layer of frosting over the top. Place the remaining cake layer on top, flattest-side up.
Optional: (If you used cake pans that are angled at the edges, refrigerate the layers for about 1 hour before trimming. There is no need to cover the cake, though you may using plastic wrap if there are strong odors in your refrigerator.) Remove the refrigerated cake and if you used cake pans that have angled sides, trim the edges of the cake so that they are straight.
Spread the remaining frosting evenly over top and sides of cake. Refrigerate until set.
Remove the cake from the refrigerator and remove the wax/parchment paper strips and serve.
Cover leftovers and store at room temperature if the room is no more than 70°F or in an air-tight container in the refrigerator. The cake may also be frozen and defrosted at room temperature.
*Please read the above-linked article on whether or not espresso and instant coffee are gluten-free.
**Please note that the video of the texture represents the use of only 5 egg yolks, making it lighter in its yellow color.
https://glutenfreerecipebox.com/gluten-free-zebra-cake/