Prep your pan: Heat the oven to 350°F (175°C). Line a 9x13-inch baking pan with parchment paper, leaving a slight overhang, or lightly grease it with cooking spray.

Melt the butter: Melt the butter and let it cool for a few minutes. Don’t add it while piping hot or it can scramble the eggs.

Mix the wet ingredients: In a large bowl, whisk the eggs and vanilla until smooth. Pour in the cooled melted butter and mix until combined.

Add the cake mix: Sprinkle in the cake mix and stir with a spatula or wooden spoon. The dough will be thick—more like cookie dough than cake batter.

Fold in the chocolate chips: Add the chocolate chips (and nuts, if using). Stir just until evenly distributed.

Press into the pan: Transfer the dough to the prepared pan. Use a spatula or your hands (lightly greased) to press it into an even layer.

Bake: Bake for 18–23 minutes, until the edges are set and lightly golden and the center looks just barely done. A toothpick should come out with a few moist crumbs.

Cool and slice: Let the bars cool in the pan for at least 20–30 minutes. Lift them out using the parchment and cut into squares.

Optional finish: Sprinkle a pinch of flaky salt on top while still slightly warm for a sweet-salty kick.