Line a 9-inch square pan with parchment paper and set aside.
Melt the butter and brown sugar in a large saucepan over medium heat.
Turn the heat down to medium-low and cook the sugar mixture for 2 to 3 minutes then remove from heat and stir in the vanilla, salt, and oats.
Pour half of the oatmeal mixture into the prepared baking dish and press it down evenly. Reserve the other half for the second layer.
For the Filling:
Place the Nutella and chocolate chips in a small microwave-safe bowl and cook until melted (stop the microwave every 15 seconds to stir well).
Set aside a few tablespoons of the melted Nutella for later. Pour the rest of the Nutella mixture over the crust in the pan and spread it evenly.
Scatter the remaining oatmeal mixture over the Nutella layer, pressing in gently. Drizzle with the reserved Nutella mixture on top.
Refrigerate 1 to 2 hours or overnight. Bring them to room temperature before slicing.
Video
Notes
The final texture of the Nutella oat bars depends on how long you boil the sugar mixture. If it doesn’t cook for very long, the bars will be soft and tend to fall apart, if you boil too long, they could turn out hard and be difficult to slice.