Preheat oven to 350 °F (175 °C). Grease and line two 9‑inch round cake pans.
In a medium bowl, sift flour, cocoa, baking powder, baking soda, and salt. Set aside.
Melt chocolate with cream in a double boiler or microwave until smooth; allow to cool somewhat.
In a large bowl, whisk eggs, milk, oil, and vanilla.
Add dry ingredients gradually to wet ingredients, mixing until combined.
Slowly pour in the melted chocolate (cooled) while stirring.
Pour in boiling water gradually (batter will be thin), mix until uniform.
Divide batter into the pans and bake 30–35 minutes, or until a toothpick comes out clean.
Cool 10 minutes in pans, then invert to rack and cool completely.
To make frosting: beat butter, then add melted chocolate mixture, then powdered sugar, vanilla, salt, adjust consistency.
Frost and assemble layers, then slice and serve.