1 and 1/4 cup (7 ounces / 200g) dark chocolate chips
1/4 cup (3.5 ounces / 100g) hazelnut butter
Line an 8-inch (or smaller for thicker bars) square baking pan with parchment paper. Set aside.
In a large mixing bowl, mix all the oatmeal cookie ingredients together using a wooden spoon. The dough should be soft and slightly sticky. Transfer half the mixture into the prepared baking pan and spread it evenly. Press very firmly using your hands. You can also top with parchment paper and press down with the bottom of a glass. Refrigerate at least 1 hour. Cover the other half of the mixture with plastic film and set aside.
In the meantime, prepare the chocolate hazelnut filling. Melt the dark chocolate over a double boiler. Once fully melted, transfer to a bowl and add the hazelnut butter. Whisk well until fully combined. Pour onto the chilled oatmeal cookie layer. Refrigerate for at least hour, or until firm. Once the chocolate hazelnut layer is firm to touch, remove from the refrigerator and let sit at room temperature 30 minutes, the chocolate will become slightly softer and will stick better to the oatmeal cookie layer.
Finally, top the chocolate hazelnut layer with the remaining oatmeal mixture. Press very firmly into an even layer. Refrigerate for another hour before cutting into 9 small bites, or bigger squares.
The bars will keep in an airtight container, at room temperature for up to 2 weeks. For firmer bars, keep them in the refrigerator.