Line 9 x 13 pan with nonstick foil.

Preheat oven to 375º.

Melt butter and chocolate together in microwave, using 30 second increments, stirring frequently as to not over heat chocolate until melted and smooth. Set aside.

Combine eggs, vanilla and sugar in large bowl (add a large pinch of salt if using unsalted butter). Using mixer, beat on high, scraping a few times, till very light and fluffy, for up to 10 minutes.

Add chocolate and mix in on low, scraping sides of bowl occasionally.

Sift flour into bowl and mix till just combined.

Spoon half of batter into prepared pan.

Smooth the batter to cover bottom of pan.

Place the candy bars side by side over batter, cutting the 4th bar into pieces to finish covering the batter.

Top with the rest of the batter and smooth the top.

Bake for 35 minutes.

Cool completely, then remove from pan using foil to lift.

Chill, cut and serve.