Lightly grease a 9×13-inch baking dish. Spread half of the salted roasted peanuts evenly over the bottom, reserving the other half for the top.

Place the butter and peanut butter chips in a large, heavy-bottomed saucepan over low heat. Stir frequently until the chips have melted into the butter. Keep the heat low to avoid scorching.

Stir in the sweetened condensed milk, mini marshmallows, and vanilla extract. Keep the saucepan over low heat and stir as the marshmallows begin to soften.

Continue stirring gently until the marshmallows are fully melted and the filling is smooth, with no white pieces remaining. Scrape along the bottom and sides of the pan as you stir. Remove from the heat; do not bring the mixture to a hard boil.

Pour the warm filling over the peanut base. Use a silicone spatula to spread it evenly to the corners, working gently so the bottom layer of peanuts stays in place.

Scatter the reserved peanuts evenly over the warm filling. Press them in lightly with the back of a spatula so they adhere without pushing the filling out of the pan.

Refrigerate for about 2 hours, or until the filling is firm enough to hold its shape when cut. Give the bars more time if the center still feels soft.

Cut into 16 bars, making four rows and four columns. Wipe the knife between cuts as needed. Serve chilled, or let the pieces sit briefly at room temperature for a softer bite.