Preheat the oven to 350°F (175°C). Grease and flour two 9-inch round cake pans and set aside.
In a large mixing bowl, sift together the all-purpose flour, cocoa powder, granulated sugar, baking powder, baking soda, and salt.
Add the eggs, whole milk, vegetable oil, and vanilla extract to the dry ingredients. Beat on medium speed with an electric mixer until well combined, about 2 minutes.
Reduce the mixer speed to low and carefully add the boiling water to the batter. Mix until fully incorporated. The batter will be thin; this is normal.
Divide the batter evenly between the prepared cake pans.
Bake for 30 to 35 minutes or until a toothpick inserted into the center comes out clean.
Remove the cakes from the oven and allow them to cool in the pans for 10 minutes. Then, remove from pans and transfer to a wire rack to cool completely.
To make the frosting, beat the softened butter on medium speed until creamy, about 2 minutes.
Gradually add the powdered sugar and cocoa powder, mixing on low speed until combined.
Add salt, vanilla extract, and heavy cream. Beat on high speed for about 3 minutes until the frosting is light and fluffy.
If the frosting is too thick, add a little more heavy cream, one tablespoon at a time. If too thin, add a little more powdered sugar.
Once the cakes are completely cool, place one cake layer on a serving plate. Spread a layer of frosting evenly over the top.
Place the second cake layer on top and apply a thin crumb coat of frosting over the entire cake. Chill for 15 minutes.
Apply the final layer of frosting smoothly over the cake.
Serve immediately or store covered at room temperature for up to 2 days or in the refrigerator for up to 5 days.