Line a 12-count muffin tin with paper liners or grease lightly with non-stick spray.

In a large saucepan over low heat, melt the unsalted butter and 1/2 cup peanut butter together until smooth.

Add the mini marshmallows to the saucepan and stir continuously until completely melted and integrated into the peanut butter mixture.

Remove the pan from the heat and gently fold in the crisp rice cereal until all grains are evenly coated.

Divide the cereal mixture among the 12 muffin cups, pressing down firmly with a greased spoon to create a level base.

In a microwave-safe bowl, combine the chocolate chips, the remaining 1/4 cup peanut butter, and the coconut oil.

Microwave the chocolate mixture in 30-second intervals, stirring between each, until the texture is completely smooth and glossy.

Spoon the chocolate mixture evenly over the rice cereal bases in the muffin tin, smoothing the tops with a spatula.

Refrigerate the tin for at least 30 minutes or until the chocolate topping has fully set and hardened.

Remove the cups from the muffin tin and serve at room temperature.