In a bowl, combine the yellow cake mix with the melted butter to form a slurry.
In another bowl, whisk together one egg, the sweetened condensed milk, and vanilla until well mixed.
Stir in the chocolate chips and toffee bits until everything is evenly combined.
Pour this mixture over the yellow cake base, making sure it is fully covered.
Bake in a preheated oven at 350°F for approximately 30 minutes.
