Heat oven to 350°F. Coat an 8-inch-round cake pan with cooking spray; line bottom with waxed paper; coat paper.
In medium-size bowl, whisk flour, cocoa powder, baking soda, baking powder and salt.
In large bowl, with electric mixer, beat butter and sugar until blended, about 1 minute. Add egg and vanilla, beat on high speed until smooth, 1 to 2 minutes. On low speed, beat in flour mixture alternating with buttermilk, until batter is blended. Evenly spread into prepared pan.
Bake in 350°F oven for 25 minutes until toothpick inserted in center comes out clean. Let cool in pan on wire rack 5 minutes. Run knife around edge; invert cake onto rack; let cool.
In medium-size bowl, combine pudding mix and skim milk. Beat on medium speed for 2 minutes until thickened. Fold in thawed whipped topping. Refrigerate 5 minutes.
Place cake on serving plate; frost sides and top with frosting. If desired, run a cake comb around sides to decorate. Serve immediately or refrigerate until serving.