Preheat Oven and Prepare Cake Pan - Preheat the oven to 350 degrees Fahrenheit. Line the cake pan with parchment paper and grease it with oil.
Whisk the Dry Ingredients - In a medium bowl, mix together the flour, cocoa powder, and baking soda with a whisk. Set the dry ingredients aside.
Mix Together with the Wet Ingredients - In a small bowl, mix the egg, sugar, oil, sour cream, coffee, and vanilla together until combined.
Combine the Wet and Dry Ingredients - Slowly add the wet ingredients to the dry products. Mix together until just combined, being careful not to overmix the batter.
Bake the Cake - Carefully pour the cake batter into the pan. Bake for 29-32 minutes, or until a toothpick inserted into the center of the cake comes out clean.
Let the cake cool in the pan for about 10 minutes. Then, use a pairing knife to remove the cake and peel off the parchment paper. Allow the cake to cool on the kitchen wire rack.