Grease a microwave-safe bowl or ramekins with nonstick cooking spray.
Combine cake mix, water, eggs, and oil in a large bowl and beat with an electric blender on low speed until moistened, about 30 seconds. Increase to medium speed and beat for 2 minutes. Stir walnuts and chocolate chips into the batter. Pour half of the batter into the prepared microwave safe bowl or ramekins.
Spoon chocolate frosting over the batter and spread evenly over the top. Pour the rest of the batter on top.
Microwave until a toothpick inserted in the middle of the cake comes out clean, 7 minutes for a bowl or 3 to 4 minutes for smaller ramekins. Remove from microwave and let rest, about 5 minutes. Dust with confectioners' sugar.