Prepared cake according to box directions. I like to over whip mine for an extra light and fluffy cake. Bake.
When done baking use a tooth pick to poke holes all over the cake.
Pour the whole can of sweetened condensed milk all over the cake.
Allow to soak in about 20 minutes.
Make icing by combining butter and cocoa over in a sauce pan over medium heat until butter is fully melted. Remove from heat whisk in the remaining ingredients.
Pour on top of cake using the back of a spoon to spread evenly.
Allow frosting to set before cutting.