Line a 8x8” pan with foil and spray with cooking spray.
Melt chocolate and sweetened condensed milk in a medium saucepan over medium-low heat. Stir constantly until the chocolate is melted and smooth. Turn off the heat and quickly stir in vanilla extract. Pour and spread into pan.
Working quickly, press each Kit Kat stick into the top of the fudge. Let the fudge cool to room temperature on the counter, then chill to set.