1. Preheat oven to 350°.
2. Prepare cake mix according to package directions, using unsalted butter, eggs, and 1 cup water. Pour batter into an 8-inch square metal baking pan coated with cooking spray.
3. Bake at 350° for 38 minutes or until a wooden pick inserted in center comes out clean. Cool in pan 10 minutes. Remove from pan; cool completely on a wire rack.
4. Combine milk and pudding mix in a medium bowl, stirring with a whisk 2 minutes. Cover and chill 5 minutes. Fold in 1 container whipped topping.
5. Cut cake into 1-inch squares. Layer half of cake in bottom of a 4-quart trifle bowl. Brush cake with 2 tablespoons liqueur, if desired; spread half of pudding mixture over cake. Layer half of strawberries over pudding. Reserve 1/4 cup of remaining whipped topping. Top strawberries with remaining 1 1/4 cups whipped topping. Repeat layers with remaining cake, liqueur, if desired, pudding mixture, and strawberries. Cover and chill 8 hours. Dollop center of trifle with reserved 1/4 cup whipped topping, and drizzle with chocolate syrup just before serving.