Melt the chocolate and butter: In a medium saucepan over low heat, melt the chocolate chips and butter together until smooth and fully melted. Stir constantly to prevent burning.

Add vanilla extract: Remove from heat and stir in the vanilla extract until well combined.

Mix in peanuts and marshmallows: Allow the chocolate mixture to cool slightly (just a few minutes), then fold in the salted peanuts and mini marshmallows until evenly coated.

Spread into pan: Pour the mixture into a parchment-lined square baking dish and press it gently into an even layer.

Chill until set: Refrigerate the pan for at least 2 hours, or until fully set and firm to the touch.

Cut and serve: Once set, lift out of the pan using the parchment and cut into squares.