Preheat your oven to 350°F (175°C). Grease and flour two 9-inch round cake pans.
In a large mixing bowl, whisk together the sifted all-purpose flour, sifted cocoa powder, granulated sugar, baking powder, baking soda, and salt until well combined.
In a separate medium bowl, beat the eggs, then add the whole milk, vegetable oil, and vanilla extract. Mix until smooth.
Dissolve the instant espresso powder in the boiling water. Let it cool slightly.
Add the wet ingredients mixture to the dry ingredients and mix until combined but do not overmix.
Add the sour cream to the batter and gently fold it in to add moisture and tenderness.
Slowly pour the cooled espresso water into the batter while stirring gently until the batter is smooth and thin.
Divide the batter evenly between the prepared cake pans.
Bake in the preheated oven for 28 to 32 minutes or until a toothpick inserted into the center comes out clean.
Remove the cakes from the oven and allow them to cool in the pans for 10 minutes.
Run a knife around the edges to loosen the cakes, then invert them onto wire racks to cool completely before frosting or serving.