Prepare a 9 X 13 baking pan by spraying with baking spray. Set aside.
Place cake mix, flour, cocoa, and sugar into the bowl of a stand mixer. Mix just until combined.
Add coffee, yogurt, eggs, and oil. Add vanilla extract now if using.
Mix on low speed until combined.
Turn mixer to medium speed and mix for 2 minutes.
Pour into prepared pan and place in preheated 325°F oven.
Set timer for 35 minutes. If you begin to smell the cake before the timer goes off, check the cake by sticking a toothpick in the center. If it comes out clean or with just a few crumbs but no wet batter, the cake is done and it needs to be removed from the oven.
Once the timer goes off, and you have not removed the cake already because it was done, check the cake for doneness by using the toothpick method.
If the cake is not done after 35 minutes, continue cooking the cake and checking for doneness every 5 to 10 minutes, depending on how wet the toothpick was when testing the cake.
When the cake is done, remove from oven and cool completely before frosting.
White Buttercream Icing
Place butter and shortening in the bowl of a stand mixer.
Using the blade attachment, mix on medium-high for 5 minutes.
Add powdered sugar 2 cups at a time and mix on slow until all sugar is incorporated.
Mix milk, extracts, and salt in a small bowl.
With the mixer turned off, add milk mixture to icing.
Turn mixer to slow and mix until incorporated.
Turn mixer to high and allow to run for 5 minutes.
Test icing for consistency. If you want a thinner icing, add milk 1 tablespoon at a time and mix for 1 minute.