Preheat your oven to 350°F (175°C). In a large mixing bowl, combine the chocolate cake mix, Greek yogurt (or sour cream), vegetable oil, milk, eggs, vanilla extract, and salt. Blend until smooth.
Pour the batter into the prepared 9x13 inch baking pan and bake for about 30-35 minutes or until a toothpick comes out clean. Let cool for 10-15 minutes.
Using a toothpick or skewer, poke holes all over the top of the cake.
In a medium bowl, mix the chocolate fudge pudding with the sweetened condensed milk and chocolate syrup until smooth.
Pour the pudding mixture over the cake, ensuring it seeps into the holes. Chill in the refrigerator for at least 2 hours.
In another bowl, whip the heavy cream with the confectioners' sugar and chocolate pudding mix until stiff peaks form.
Spread the whipped topping over the chilled cake. Optionally, decorate with chocolate shavings or sprinkles.
Slice into squares, serve, and enjoy!
