Preheat oven to 350°F. Grease an 8-inch square pan with butter.
In the greased pan, combine the flour, cocoa powder, sugar, baking soda, and salt. Whisk lightly with a fork or small whisk to combine. Make two small wells on the sides and 1 large well in the middle of the dry ingredients.
Pour the vinegar and vanilla into the small wells then pour the oil in the big well, if they spill over the wells that's perfectly okay.
Pour the water over the top and whisk again to combine thoroughly.
Bake for 30-35 minutes or until a toothpick inserted comes out clean. Cool completely.
Frost the cake with your choice of chocolate frosting. Optional; sprinkle rainbow sprinkles on top.