+For cake:
°1 1/2 cups all-purpose flour
°1 cup of sugar
°1/4 cup of unsweetened cocoa powder
°1/2 teaspoon salt
°1 teaspoon of baking soda
°1 teaspoon vanilla
°1 teaspoon white vinegar
°1/3 cup vegetable oil
°1 cup of water
+To decorate:
°2 tablespoons of butter
°1/4 cup of unsweetened cocoa powder
°1 cup powdered sugar
°1 tablespoon of milk (or cream)
°1/4 teaspoon of vanilla extract

*How to make 

Combine the flour, sugar, cocoa powder, salt, and baking soda in a large mixing bowl.
In another bowl, mix vanilla, vinegar, oil and water. Add the wet ingredients to the dry ingredients and mix until completely combined and there are no lumps left.
Pour the batter into a greased 8x8 square pan.
Bake in a preheated 350° oven for 30 to 35 minutes, until a toothpick inserted in the center comes out clean.
Leave the cake to cool completely before decorating it.
How to make ice
Melt the butter in a small saucepan (or in the microwave using a microwave-safe dish). Add cocoa powder to the mixture until it forms a thick paste.
Transfer the chocolate mixture to a medium mixing bowl. Using the mixer on low speed, add the powdered sugar, milk and vanilla.
Once the ingredients are combined, turn the mixer on medium-high speed and beat for about 5 minutes, until the frosting is smooth and creamy. Spread over cooled cake.

Enjoy !