Combine powdered sugar and cream cheese in a bowl, and beat at high speed of a mixer until well-blended. Gently fold 1 cup whipped topping into the cheese mixture.
Combine 1/2 cup sugar and next 2 ingredients in the top of a double boiler; place over simmering water. Beat at high speed of a mixer until stiff peaks form. Gently stir one-fourth of egg white mixture into cheese mixture. Gently fold in remaining egg white mixture; set aside.
Combine hot water and next 3 ingredients; stir well. Split the ladyfingers in half lengthwise. Arrange 20 ladyfinger halves, cut sides up, in the bottom of an 8-inch square baking dish. Drizzle half of the espresso mixture over ladyfinger halves. Spread half of cheese mixture over ladyfinger halves; repeat procedure with the remaining ladyfinger halves, espresso mixture, and cheese mixture. Spread remaining 1/2 cup whipped topping evenly over cheese mixture; sprinkle with cocoa.
Place one toothpick in each corner and in center of Tiramisù to prevent plastic wrap from sticking to whipped topping; cover with plastic wrap. Chill 2 hours.