Cream butter, peanut butter, and sugar until smooth using a stand or a hand mixer. Beat in vanilla, salt, flour, and milk.
Melt the candy melts according to package directions. Line a cookie sheet with waxed paper and spread half of the melted candy into about an 11”x8” rectangle using an offset spatula. Chill for a few minutes, so that the candy has started to harden but is still semi-soft (less than 5 minutes). You want the candy hard enough that it won’t absorb the cookie dough mixture, but still soft enough that it will adhere to the cookie dough.
Remove candy from refrigerator and spread the cookie dough evenly inside the rectangle, leaving small edges so that top candy layer will adhere to the bottom.
Spread the remaning candy on top of the cookie dough using an offset spatula, making sure that it meets the bottom layer to seal the edges. Immediately sprinkle with jimmies or other candies/sprinkles. Chill until the hardened, about 20 minutes. Break into pieces and serve.