Preheat oven to 350 degrees F. Grease a 9 x 13 inch baking pan or 5 mini cake pans.
Whisk together flour, sugar, cocoa powder, baking soda, and salt to a large mixing bowl. Add oil, coffee, vinegar, and vanilla. Whisk until smooth. Pour the batter into prepared cake pans.
If using mini cake pans, place on a large baking sheet. Bake for 35-40 minutes, or until a wooden skewer insterted into the center comes out with just a few crumbs.
Cool cake 10 minutes in pan before inverting onto a wire rack to cool completely.
To make the frosting, place the avocados into the bowl of a food processor. Blend until smooth. Add powdered sugar, cocoa powder, and vanilla. Process until combined.
Top cooled cakes with frosting and serve.