Preheat Oven:
Preheat your oven to 350°F. Grease two 8-inch round cake pans and line the bottoms with parchment paper.

Mix Dry Ingredients:
In a large bowl, whisk together flour, sugar, cocoa powder, espresso powder (if using), baking powder, and salt.

Melt Chocolate and Butter:
In a saucepan over low heat, melt butter with chopped chocolate. Stir until smooth, then let it cool slightly.

Combine Wet Ingredients:
In another bowl, whisk eggs. Add milk, sour cream, and vanilla. Mix well.

Make the Batter:
Add the melted chocolate mixture to the dry ingredients. Stir in the egg mixture. Blend until smooth.

Bake:
Divide the batter evenly into the prepared pans.
Bake for 25-30 minutes, or until a toothpick inserted in the center comes out clean.
Let the cakes cool in pans for 10 minutes, then transfer to wire racks to cool completely.

Make Ganache:
Place chopped chocolate in a bowl.
Heat cream in a saucepan until just simmering, then pour it over the chocolate.
Let sit for 2-3 minutes, then stir until smooth.

Assemble the Cake:
Place one cake layer on a plate. Spread some ganache on top.
Add the second layer and pour remaining ganache over the cake, letting it drip down the sides.
Let the ganache set for 30 minutes before serving.