Makes 24 pieces
Prepare cake mix according to package directions. Bake in foil-lined 13x9x2-inch pan; cool completely.
Poke holes all over cooled cake with skewer. Set aside. Pour chocolate milk into medium saucepan and heat over medium until bubbles begin to form around edges. Remove from heat and whisk in chocolate chips and cinnamon. Whisk in sweetened condensed milk; cool completely.
Slowly pour mixture over entire cake, poking additional holes if necessary to absorb liquid. Cover and refrigerate overnight until liquid is absorbed.
When ready to serve, whip whipping cream until stiff peaks form. Cut cake and garnish with strawberries and whipped cream.