Preheat oven to 350 degrees. Grease sheet pan with 1 tablespoon melted butter or non stick cooking spray. Set aside. In large bowl, combine all ingredients together until just mixed. Pour into prepared pan and smooth. Bake 12-15 minutes. Remove from oven and cool.
For the frosting, whip all ingredients together until well combined and smooth. Scrape edges of bowl and whip again 1-2 minutes. Frost top of cake and refrigerate until ready to serve.