Preheat your oven to 350°F (175°C). Grease a baking sheet.
In a mixing bowl, combine flour, sugar, baking soda, and salt.
In a saucepan, bring water, vegetable oil, and cocoa powder to a boil. Pour over the dry ingredients and mix well.
Add vanilla extract and mix until smooth. Pour the batter into the prepared baking sheet.
Bake for 20 minutes or until a toothpick inserted into the center comes out clean.
For the frosting, melt vegan butter in a saucepan. Stir in cocoa powder and almond milk. Remove from heat and mix in powdered sugar and vanilla extract until smooth.
Pour the frosting over the warm cake and spread evenly. Let it cool before serving.