Step 1: Preheat your oven to 350°F (175°C). Grease and flour a 9x13 inch baking pan.
Step 2: Prepare the yellow cake mix according to package directions in a large bowl. Pour the batter evenly into the prepared 9x13 inch baking pan.
Step 3: Bake for 25-30 minutes, or until a wooden skewer inserted into the center comes out clean. Do not overbake.
Step 4: While the cake is still warm from the oven, use the handle of a wooden spoon or a fork to poke holes all over the surface of the cake, ensuring the holes go deep enough.
Step 5: In a small bowl, whisk together the sweetened condensed milk and melted butter until thoroughly combined. Slowly and evenly pour this entire mixture over the warm, poked cake, ensuring it seeps into all the holes.
Step 6: Generously sprinkle the Heath English toffee bits evenly over the top of the cake.
Step 7: Allow the cake to cool completely at room temperature, then cover and refrigerate for at least 2-3 hours before serving. Chilling helps the cake set and the flavors meld. Store leftovers covered in the refrigerator.