Chop peanut butter cups and place them in the freezer. Preheat oven to 350°.
Whisk together melted butter, sugar, and cocoa. Add eggs and vanilla and whisk till smooth. Stir in flour, salt, and about 3/4 cup of the peanut butter cups.
Spread batter into a greased 9x13" pan. Drip peanut butter by spoonfuls on top of batter. Swirl with a butter knife.
Bake at 350° for about 20 minutes. Sprinkle remaining peanut butter cups on top.
Bake for another 18-20 minutes. Cool and cut into squares.