Preheat oven to 350°F (175°C). Grease and line two 8-inch round pans (or one 9x13 pan).
Whisk flour, cocoa powder, baking powder, baking soda, and salt in a large bowl.
In another bowl, whisk eggs, milk, oil, and vanilla.
Add wet to dry and mix until smooth.
Slowly whisk in boiling water (batter will be thin).
Optional: whisk in melted dark chocolate for extra richness.
Divide into pans and bake 28–35 minutes (depending on pan size), until a toothpick comes out clean.
Cool 10 minutes in pans, then move to a rack to cool completely.
Beat cream cheese + butter until smooth.
Mix in cocoa powder, powdered sugar-free sweetener, and vanilla.
Add cream 1 tbsp at a time until spreadable.
Frost the cooled cake and slice.
