Step 1
Preheat oven to 350 degrees F (175 degrees C).
Step 2
Spray 2 (8-inch) cake pans with cooking spray.
Step 3
Stir yellow cake mix 3 eggs water and vegetable oil in a bowl until moistened. Beat with an electric mixer on medium speed for 2 minutes scraping bowl occasionally.
Step 4
Divide cake batter between the two prepared cake pans.
Step 5
Bake in the preheated oven until cakes have risen and are lightly golden brown 33 to 38 minutes. Check for doneness after 25 minutes. A toothpick inserted into the center of the cakes should come out clean. Allow cakes to cool for about 10 minutes before removing from the pans. Cool cakes completely about 40 minutes.
Step 6
Whisk 3 eggs cornstarch and sugar in a large bowl until well mixed and lemon colored 2 to 3 minutes. Set aside.
Step 7
Heat 1 cup heavy whipping cream whole milk and 1/2 tablespoon butter in a large saucepan over medium heat until it just begins to boil.
Step 8
Reduce heat to low; pour egg mixture into cream mixture and whisk until thick about 1 minute.
Step 9
Pour egg and cream mixture through a fine mesh strainer into a large bowl; discard any chunks.
Step 9
Whisk vanilla and salt into the egg and cream mixture until combined. Cover with a layer of plastic wrap pressed against the surface. Chill in refrigerator to form a pastry cream texture 3 to 4 hours.