Preheat oven to 180°C/350°F/160°FF and line 4 baking trays with baking paper
In the bowl of a stand mixer, beat the eggs, sugars and salt on medium-high for 8 minutes, making the mixture light and fluffy
While the eggs and sugars are mixing, combine the flour and baking powder in a small bowl and set aside
In a heatproof bowl, melt the dark chocolate and butter over simmering water, stirring to combine
Remove chocolate from the heat and stir through the cocoa until mixture is smooth
Once the eggs, sugars and salt have mixed for 8 minutes, reduce speed of the mixer and add vanilla and canola. Mix to combine
Now add the chocolate mixture and mix on low until just combined
Stop the mixer and add the flour and baking powder. Mix until just combined
Stir through the chocolate chips
Allow the mixture to sit at room temperature for 15 minutes. It will thicken as it cools
Using a small ice cream scoop (2 teaspoons) drop heaped scoops of mixture onto lined trays, leaving space for the cookies to spread a little. Bake cookies for 6 minutes, then remove from oven. Straight away, use a round cookie cutter to scoot the cookies back into round shapes if needed. Slide baking paper off tray and onto the bench and allow to cool, before placing them on a cooling rack to cool completely
Once completely cool, pipe the nutella filling onto half of the cookies, sandwiching with another cookie