PREHEAT oven to 300°F
EMPTY contents of cake mix pouch into large bowl. BLEND cake mix, egg, water and softened butter in a large bowl with an electric mixer at low speed for 30 seconds. Scrape contents of bowl. BEAT at medium-high speed for 3 minutes. POUR batter into the UNGREASED pan and spread batter evenly.
BAKE on rack in center of oven at 300°F for 34-39 minutes. Cake is done when toothpick inserted in center comes out free of crumbs. REMOVE cake carefully from the over with both hands using pot holders. COOL cake in pan for 15 minutes before removing.
REMOVE cake by turning the pan over and gently pushing on the bottom center of the pan to release the cake. CAREFULLY peel off the cake pan disk and place the cake right side up on a wire rack. Cool completely before frosting.
PLACE softened butter into large bowl with high sides. MIX softened butter with electric mixer on high for 1 minute or until light and fluffy. Scrape.
ADD frosting mix and exactly one teaspoon of water.
STIR with a rubber spatula or spoon, pressing against the side of the bowl, until powder is incorporated into butter and mixture becomes dark brown and well blended. Scrape sides. MIX with an electric mixer on high for 2 minutes or until smooth and creamy. For a creamier consistency, add up to an additional ½ teaspoon of water.
FROST cake with spatula.
*Don't cooking in a microwave or toaster oven.