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Prepare Cake Batter: In a large bowl, milk and oil are combined. An egg is cracked into the bowl. Vanilla extract is added. The wet ingredients are whisked together until combined. Cocoa powder and sugar are added to the wet ingredients. The mixture is whisked until it forms a thick, smooth chocolate batter. Espresso is brewed and then poured into the chocolate batter. The batter is mixed thoroughly with a spatula until well combined and appears smooth and pourable.