Heat oven to 350°F (175°C). Line a 9×13 pan with parchment, leaving overhang for easy lifting. Lightly mist with nonstick spray for minimal cleanup.

In a bowl, combine cake mix, melted butter, and egg until a soft, crumbly dough forms.

Reserve ¾–1 cup of crumbs for the topping; this keeps the finish chewy with light crunch.

Press the remaining dough evenly into the pan. Pack the edges so they don’t over-brown.

Bake 8–10 minutes until the surface looks dull and the edges are just turning golden. This quick par-bake prevents a soggy base.

Stir the sweetened condensed milk to loosen, then drizzle it over the warm crust, spreading to the corners.

Sprinkle toffee bits and chocolate chips evenly for that chewy caramel crunch.

Scatter the reserved crumbs over the top. Aim for small pebbles rather than a solid sheet so pockets of chocolate and toffee peek through.

Return to the oven for 14–18 minutes, until the edges are deep golden and the center looks set but still soft. If the top browns too fast, tent loosely with foil for the last 3–5 minutes.

Place the pan on a wire rack and cool fully—the bars firm as they cool. For ultra-clean slices, chill 30–60 minutes.

Lift out using the parchment overhang. Sprinkle flaky sea salt while the surface is faintly warm so it adheres.

Use a warm, dry knife to cut into 16–24 bars; wipe the blade between cuts for neat edges.

Doneness check: Gentle jiggle in the center is fine; it will set during cooling. Dry edges and light bubbling around the sides mean you’re there.

Pan options: For an 8×8 or 9×9 pan, reduce all ingredients by about one-third and add 3–5 minutes to the final bake if needed for thickness.

Texture control: Shorter bake = softer, gooier bars. Longer bake = firmer, cleaner slices.

Storage: Room temperature up to 3 days in an airtight container; refrigerate up to 5 days; freeze up to 2 months with parchment between layers.