Preheat your oven to 350 degrees. Spray an 8x8 with cooking spray and set aside.
In a large bowl, mix together the flour, cocoa powder, baking soda, baking powder, and salt.
In a medium bowl whisk together the vegetable oil, sugar, brown sugar, vanilla extract, and eggs. Stir into the flour mixture. Then stir in the 1 cup of Horizon Organic Milk, until just smooth.
Pour into the prepared pan and bake for 35-38 minutes or just until a toothpick comes out clean.
Allow to cool.
In a medium bowl, mix together the butter, powdered sugar, cocoa powder, and Horizon Organic Milk until smooth. Spread over cooled cake and serve.