Preheat the oven to 350°F (175°C). Grease and flour a 9x13-inch baking pan or line it with parchment paper.
In a large mixing bowl, combine the cake mix, softened butter, and eggs. Mix well until the batter is smooth and fully combined.
Fold in the toffee bits (and any optional add-ins like chocolate chips or nuts).
Spread the batter evenly into the prepared baking pan. Smooth the top with a spatula to ensure an even layer.
Bake in the preheated oven for 25-30 minutes or until the bars are golden brown and a toothpick inserted into the center comes out clean.
Allow the bars to cool completely in the pan before cutting them into squares.