Preheat oven to 350 degrees F. Line a baking sheet with parchment paper.
In a medium bowl, whisk together oats, flour, baking soda, cinnamon, salt. In the bowl of an electric mixer, cream the butter, Nutella, and brown sugar until smooth. Add the egg and vanilla and beat until fluffy.
Turn the mixer to low and gradually add in the oat mixture and mix to combine. Stir in the toffee and chocolate chunks.
For best results, chill the dough for 2 hours or for 48 hours. The dough can be chilled in the mixing bowl or scooped into balls and chilled.
When ready to bake, place cold dough balls at least 2 inches apart on a baking sheet lined with a silicon baking mat or parchment paper. I use a spring-release scoop that holds 4 tablespoons (¼ cup) and bake them for 16 minutes. Cooking time will vary depending if you use a light or dark pan and the size of your cookies. Watch them carefully for the degree of doneness you like. You can test by baking one cookie the first time and adjust the timing for the remaining..
Cool on the baking sheet for 2 minutes before transferring to a wire rack to cool completely.
Store in an air-tight container at room temperature for up to one week.