Krispie Treats:

Line an 8 x 8-inch square tin with parchment, and spray generously with nonstick spray.

Add the butter to a large, heavy-bottomed saucepan or stock pot. Melt the butter.

Stir in the Nutella and allow it to melt completely.

Stir in the marshmallows, and continue stirring until completely melted. Remove from heat.

Stir in the puffed rice cereal, and continue stirring until well combined, with no dry cereal remaining.

Pour cereal mixture onto prepared tin.

With damp fingertips, gently* press down to distribute the mixture evenly across the surface of the tin.

Set aside and allow to cool at room temperature for at least 1 hour, or until cereal mixture is cool to the touch.

Ganache Topping:

Melt the dark chocolate over a bain marie or double boiler.

Meanwhile, scoop the Nutella into a microwave-safe bowl. Microwave on medium power for 15-30 seconds, stirring occasionally, until warm throughout.

Whisk the warm Nutella into the melted chocolate.

Pour ganache topping over the cereal mixture and use a rubber spatula to spread.

Set aside and allow to cool at room temperature for at least 30 minutes more, or until ganache sets. Slice into squares and serve.

Layer leftover Nutella Rice Krispie treats between sheets of parchment. Store in an airtight container at room temperature for up to three days.