Preheat the oven to 350°F. Spritz a 9 x 13 inch baking pan with cooking spray. Set aside.
In a heavy bottomed saucepan met together the butter and brown sugar. Cook, stirring for 2 minutes or until the sugar has dissolved. Cool to warm.
Beat the vanilla and the eggs into the cooled brown sugar and butter one egg at a time.
Sift together the flour, baking powder and salt. Gradually add to the bowl, beating with a hand mixer until fully combined.
Stir 1/3 of the chopped Snickers and 1/2 of the chopped peanuts into the batter by hand.
Pour the batter evenly into the prepared pan. Sprinkle the remaining Snickers bars and chopped peanuts on top.
Bake for 35-40 minutes or until a toothpick inserted into the center shows moist crumbs. Cool on a cooling rack in the pan. Cut into desired size squares.