1. Prepare the Oven and Pan:

1. Preheat oven to 350°F (175°C).
2. Grease a 9×13-inch baking pan or line it with parchment paper, leaving an overhang on the long sides for easy removal.

2. Make the Crust and Topping Mixture:

1. In a large bowl, combine the yellow cake mix, melted butter, and egg.
2. Mix with a spatula or wooden spoon until well combined and the mixture is crumbly. It will resemble coarse crumbs.

3. Form the Crust:

1. Set aside about 1/3 of the cake mix mixture for the topping.
2. Press the remaining 2/3 of the mixture evenly into the bottom of the prepared baking pan to form the crust.

4. Add the Filling:

1. Pour the sweetened condensed milk evenly over the crust, spreading it gently with a spatula to cover the entire surface.
2. Sprinkle the chocolate chips evenly over the sweetened condensed milk.
3. Sprinkle the toffee bits evenly over the chocolate chips.

5. Add the Topping:

1. Crumble the reserved cake mix mixture evenly over the top of the toffee bits.

6. Bake:

1. Bake for 25-30 minutes, or until golden brown and set. The edges should be slightly crispy, but the center may still look a bit gooey. The internal temperature should be around 200F. If using a glass pan, reduce the temperature to 325F.

7. Cool and Slice:

1. Remove the pan from the oven and let the bars cool completely in the pan before slicing. This is important, as the bars will be very soft when warm and will firm up as they cool.
2. Once cooled, lift the bars out of the pan using the parchment paper overhang (if used) or carefully cut them into squares within the pan.