Line a 9-inch square baking dish with parchment paper and set it aside. I use binder clips to keep the parchment paper in place.
In a large pot, melt butter over medium heat. Once melted, reduce heat to low and add all but 1 cup of the mini marshmallows to the pot.
Stir until marshmallows are completely melted. Remove from heat.
Add in vanilla extract and 1 cup of the Nutella. Stir until smooth. Add cereal and remaining 1 cup of the mini marshmallows. Stir until cereal is completely covered in marshmallow mixture, then pour it into prepared pan.
Melt remaining 1/4 cup of Nutella in micrwoave for 30 seconds. Drizzle over krispie treats. Allow treats to set about one hour. Cut and enjoy.
Notes
Unsalted butter can also be used in place of the salted butter. Add about 1/2 teaspoon kosher salt to butter and marshmallow mixture.
Always use fresh marshmallows and fresh cereal for the best results.
Nutella hazelnut spread can be found at most grocery stores or ordered online. Swap it out with Biscoff, almond butter, or peanut butter if you prefer.
For best texture, do not press the bars TOO firmly into the pan. Just lightly press with back of spatula or fingers dipped in water (to prevent sticking).
Store treats in airtight container at room temperature for up to 5 days. Or freeze in airtight ziploc bag for up to 3 months (thaw on counter for several hours).