Preheat your oven to 350F. Grab a 9x13-inch baking pan and spray it with cooking spray to prevent sticking.
In a large mixing bowl, combine all your ingredients: the chocolate cake mix, chocolate fudge instant pudding mix, Greek yogurt (or sour cream), vegetable oil, milk, eggs, vanilla extract, and salt. Mix until everything is well combined.
Now, pour the batter into your prepared pan and spread it evenly. Bake for about 28 to 32 minutes, or until a toothpick inserted in the center comes out clean or with a few moist crumbs.
Once baked, allow the cake to cool in the pan for about 30 minutes.
Using the tines of a fork, poke holes all over the cake, going down nearly to the base.
In a medium bowl, combine the sweetened condensed milk and chocolate syrup. Whisk them together until fully blended.
Evenly pour the filling mixture over the cake.
In another large mixing bowl, add the heavy cream, confectioners' sugar, and the chocolate fudge instant pudding mix. Beat until fluffy and spreadable.
Evenly spread the frosting over the top of the cake, cover, and refrigerate until serving.