Preheat your oven to 350°F (175°C). Grease and flour two 9-inch round cake pans or line with parchment paper.

In a large bowl, sift together the flour, cocoa powder, baking powder, baking soda, and salt.

In another bowl, whisk together the milk, oil, vanilla extract, and eggs until smooth. Add the sugar and beat until combined. Stir in the sour cream.

Gradually add the dry ingredients to the wet ingredients, mixing just until incorporated. Slowly stir in the boiling water (batter will be thin this is normal).

Divide the batter evenly between the prepared pans. Bake for 30–35 minutes, or until a toothpick inserted into the center comes out clean. Let the cakes cool in the pans for 10 minutes, then transfer to wire racks to cool completely.

In a small saucepan, heat the cream and butter over medium heat until it just begins to simmer. Remove from heat and add the chopped dark chocolate. Let sit for 2–3 minutes, then stir until smooth and glossy. Let the frosting cool slightly until it thickens to a spreadable consistency.

Place one cake layer on a serving plate. Spread a layer of chocolate ganache on top. Add the second cake layer and frost the top and sides with the remaining ganache. Let set for 15–20 minutes before serving for best results.