Place a small piece of graham cracker at the bottom of each cavity in a silicone ice cube tray. Add 1-2 mini marshmallows on top. Set aside.
In a microwave-safe bowl, melt the dark chocolate with coconut oil in 30-second intervals, stirring until smooth. Mix in the peanut butter until well combined.
Pour the chocolate mixture into each cavity of the ice cube tray, smoothing out the tops.
Freeze for at least 4 hours until firm to the touch.
Remove the fudge from the tray and enjoy!