Heat oven to 350°F. Line an 8-inch square pan with parchment, leaving an overhang.
Whisk melted butter, vanilla, and the beaten egg in a large bowl.
Add cake mix and mix until a thick, soft dough forms.
Press about two-thirds of the dough evenly into the prepared pan.
Sprinkle half the chocolate chips over the base.
Drizzle the sweetened condensed milk evenly over the chips.
Scatter remaining chips, then pinch off and dot the rest of the dough over the top, leaving small gaps.
Bake 24–28 minutes until edges are golden and the center looks just set but still soft.
Cool completely in the pan. Chill 30 minutes for cleaner slices.
Lift out using the parchment, cut into 16 bars, and finish with flaky sea salt if using.
Notes
Variation: Swap 1/2 cup of the chocolate chips for butterscotch or peanut butter chips, or fold in 1/2 cup chopped pecans for crunch. Storage: Keep covered at room temperature for 2–3 days, or refrigerate up to 1 week. Freeze well-wrapped bars for 2 months.This recipe is an original creation inspired by classic Decadent Chocolate Chip Gooey Bars flavors. All ingredient ratios and instructions are independently developed.