Prepare the Cake:

1. Preheat the Oven: Set to 350°F (175°C). Grease and line two 9-inch cake pans with parchment paper.
2. Mix Dry Ingredients: In a large bowl, whisk together flour, sugar, baking powder, baking soda, salt, and cocoa powder.
3. Mix Wet Ingredients: Add milk, vegetable oil, eggs, and vanilla extract to the dry mixture and mix until just combined.
4. Add Boiling Water: Slowly stir in boiling water, mixing until smooth. The batter will be thin—this is normal.
5. Bake: Divide the batter evenly between the cake pans and bake for 30-35 minutes, or until a toothpick inserted in the center comes out clean.
6. Cool the Cakes: Let them cool in the pans for 10 minutes, then transfer to a wire rack to cool completely.

Prepare the Chocolate Frosting:

1. Melt the Butter: In a saucepan or microwave, melt butter and whisk in cocoa powder until smooth.
2. Mix Ingredients: In a large bowl, combine powdered sugar, milk, vanilla extract, and salt with the butter mixture. Beat until smooth and fluffy. Adjust milk as needed for the desired consistency.

Assemble the Cake:

1. Frost the Layers: Place one cooled cake layer on a serving plate and spread an even layer of chocolate frosting on top.
2. Stack and Frost: Place the second cake layer on top and spread the remaining frosting over the top and sides.
3. Set Before Serving: Let the frosting set before slicing and serving.