Preheat oven to 350˚F. Oil and flour a 12 cup bundt pan.
For cake; mix sugar, softened butter and vanilla in a mixing bowl; blend in cocoa, eggs, and sour cream.
Add milk slowly while mixing at low speed; gradually add Sopaipilla Mix while continuing to mix until batter is smooth. Pour into prepared pan.
Bake 45 to 50 minutes, or until toothpick inserted into center comes out clean. Remove from oven.
Invert cake onto wire rack to cool immediately after removing from oven. Cool completely before frosting.
Frost with chocolate frosting, or dust with powdered sugar.
For chocolate frosting; mix butter, vanilla, and cocoa in a medium sauce pan; cook over medium heat to boiling, stirring constantly.
Remove from heat; stir in powdered sugar. Blend thoroughly; stir in vanilla.