- In a small saucepan, combine the butter, water, and 1/4 cup cocoa powder over low heat. Remove from heat and pour into a large heat-resistant bowl to cool.
- Add flour, granulated sugar, eggs, sour cream, baking soda and salt; beat until well combined then pour into a 10" x 15" rimmed baking sheet.
- Bake 25 minutes, or until a knife inserted in the center comes out clean; set aside to cool.
- In a small saucepan, make frosting by melting margarine with milk and remaining 2 tablespoons cocoa powder over low heat. Remove from heat and stir in confectioners’ sugar and vanilla; mix well then frost cooled cake.