Preheat your oven to 350°F (175°C). Grease two 9-inch round cake pans and dust them with flour.
In a large mixing bowl, whisk together the flour, sugar, cocoa powder, baking powder, baking soda, and salt.
Add eggs, milk, melted butter, and vanilla extract to the dry ingredients and mix until smooth.
Slowly stir in the boiling water until the batter is combined and thin.
Divide the batter evenly between the prepared pans and bake for 30-35 minutes, until a toothpick comes out clean.
Let the cakes cool in the pans for 10 minutes before transferring them to wire racks to cool completely.
For the frosting, beat the softened butter and cocoa powder until smooth. Gradually add powdered sugar, cream, vanilla extract, and a pinch of salt until fluffy.
Once cooled, spread frosting between the layers, on top, and around the sides of the cake.
Notes
For a dairy-free option, swap whole milk with almond or oat milk and use coconut oil instead of butter.You can enhance the chocolate flavor of the frosting by adding a teaspoon of instant espresso powder.Store leftovers in an airtight container at room temperature for up to 2 days, or refrigerate for longer freshness. Individual slices can be frozen for up to 3 months.