Preheat oven to 325°F. Lightly grease and dust with flour a round 8-inch cake pan. Set aside.
Combine the flour, sugar, cocoa powder,and baking soda in a large mixing bowl. Stir with a whisk until well blended.
Combine in a seperate bowl the 1 cup water along with the oil, vanilla extract and vinegar. Whisk until blended. Pour liquid mixture into the flour and combine well with a spoon or spatula.
Pour the mixture into the prepared baking pan. Smooth the top and tap pan a couple of times on the counter to release air bubbles.
Place the dish to the oven and bake until a knife inserted into the center of the batter comes out clean,about 28 to 33 minutes.
Remove cake from the oven, and let cool in the pan. Once cooled cut into 8 pieces. Sprinkle icing sugar on top and serve.