Prepare cake mix according to package directions using an 8-inch square or round cake pan. If desired, line cake pan with parchment paper; continue as directed. Cool cake in pan on a wire rack for 10 minutes; remove cake from pan and remove parchment if used. Cut cake into 1-inch pieces.
Meanwhile, prepare pudding mix according to package directions using the 2 cups fat-free milk; cover and chill about 30 minutes or until set.
Place half of the cake cubes in a 3-quart trifle bowl. Spoon half of the cherries over cake; spread half of the pudding over cherries and top with half of the whipped topping. Repeat layers. If desired, sprinkle with cocoa powder.
To make ahead: Prepare as directed. Cover and chill for up to 4 hours.