Set your oven to 350°F (175°C). Grease and flour two 9-inch round cake pans and set them aside.
In a large bowl, whisk together sugar, flour, Hershey’s Cocoa, baking powder, baking soda, and salt.
Add eggs, milk, vegetable oil, and vanilla extract to the bowl. Beat on medium speed for 2 minutes until the batter is smooth.
Slowly stir in the boiling water. The batter will be thin—this helps keep the cake moist.
Divide the batter evenly between the two pans. Bake for 30–35 minutes, or until a toothpick inserted in the center comes out clean. Cool in pans for 10 minutes, then transfer to wire racks to cool completely.
In a separate bowl, beat softened cream cheese, powdered sugar, and vanilla extract until smooth and creamy.
Beat softened butter and cream cheese until fluffy. Slowly mix in cocoa powder and powdered sugar. Stir in vanilla, then add milk 1 tablespoon at a time until the frosting is smooth and spreadable.
Slice each cake in half horizontally. Place one layer on a serving plate and spread one-third of the cream cheese filling on top. Repeat with the next two layers. Add the final cake layer and frost the entire cake with the chocolate buttercream.