Preheat the oven to 350°F (175°C). Grease and flour two 9-inch round cake pans and line the bottoms with parchment paper.
In a large mixing bowl, sift together the flour, cocoa powder, sugar, baking powder, baking soda, and salt.
In a separate medium bowl, whisk the eggs, milk, vegetable oil, vanilla extract, and sour cream until smooth.
Gradually add the wet ingredients to the dry ingredients, mixing until just combined.
Slowly pour in the hot espresso while stirring gently to combine the batter evenly.
Divide the batter evenly between the prepared cake pans and smooth the tops with a spatula.
Bake for 30 to 35 minutes or until a toothpick inserted in the center comes out clean.
Remove the cakes from the oven and allow them to cool in the pans for 10 minutes before transferring to a wire rack to cool completely.
For the ganache, place the chopped dark chocolate in a heatproof bowl.
Heat the heavy cream in a small saucepan over medium heat until it just begins to simmer.
Pour the hot cream over the chopped chocolate and let it sit for 2 minutes, then stir gently until smooth and glossy.
Allow the ganache to cool slightly until it thickens but is still pourable.
Place one cake layer on a serving plate and spread a thin layer of ganache over the top.
Place the second cake layer on top and pour the remaining ganache evenly over the top and sides, smoothing with a spatula.
Let the ganache set at room temperature for about 30 minutes before serving.