Line 9×13 pan with foil and spray with nonstick spray. Set aside.
In the bowl of stand mixer fitted with the paddle attachment combine the butter, sugar, vanilla and eggs. Mix on medium speed until combined, about 1 minute.
Add in salt and the cocoa powder and mix on medium until combined, 30 seconds to a minute.
Turn the mixer to low and add in the flour until incorporated.
Stir in the chocolate chips.
Spread half of the batter into pan. Top with the candy bars, breaking to fit if necessary, and spread the remaining batter on top.