Preheat your oven to 350°F (175°C). Grease and flour a 9×13 inch baking pan.

In a large bowl, combine the yellow cake mix, water, vegetable oil, and eggs. Mix on low speed until just combined, then increase speed to medium and mix for 2 minutes until smooth.

Pour the batter into the prepared pan and bake for 25-30 minutes, or until a toothpick inserted into the center comes out clean.

Remove the cake from the oven and immediately poke holes all over the warm cake using a fork or the end of a wooden spoon. Make sure to go all the way down and make around 100 to 150 holes.

While the cake is baking, prepare the sauce. In a medium saucepan, combine the sweetened condensed milk, heavy cream, granulated sugar, and melted butter.

Heat the sauce over medium heat, stirring constantly, until the sugar is dissolved and the mixture is smooth and just starts to simmer. Turn off the heat and stir in the vanilla extract.

Slowly pour the warm sauce evenly over the warm poked cake, making sure to get sauce in all of the holes.

If desired, top with sprinkles immediately after pouring the sauce.

Let the cake cool completely before serving. For best results, chill for at least 2 hours before serving, as this allows the sauce to fully absorb into the cake.