Nutella filling: Measure out 12 tablespoons of Nutella onto a plate covered with a piece of baking paper and place in the freezer, until ready to use.
Cookie dough: Using an electric hand mixer or a stand mixer fitted with paddle attachment, cream the softened butter and both sugars together on medium speed until light and fluffy, approx. 2 minutes.
Add the egg and beat on medium speed until combined. Scrape down the sides as needed and then add the vanilla and beat again until completely incorporated.
Add the flour, bicarbonate of soda, cream of tartare, cinnamon and salt to the bowl and beat together until well combined.
The dough is quite sticky, but don't be tempted to add more flour. Prepare a plate or tray that will fit in your fridge, covered with a piece of baking/parchment paper. Using a 2tbsp ice cream scoop or a large spoon, scoop out 12 balls of cookie dough on to your prepared tray. Place the dough in the fridge for 20-30 minutes to firm up.
Preheat your oven to 170°C fan / 375°F / Gas 5 and prepare 2 large baking trays, lined with baking/parchment paper.
Take a ball of cookie dough and flatten the dough slightly into a disc shape. Add a frozen piece of Nutella to the centre and then wrap the cookie dough around the Nutella so its covered and becomes a ball again.
Topping: To crush the nuts, place them in a plastic bag and bash them with a rolling pin. Then combine with the sugar and cinnamon in a large bowl.
Roll the cookie dough balls in the topping, push the dough slightly into the mix to make sure you pick up lots of topping.
Place 6 pieces onto each prepared baking tray, leaving space between to allow for spreading in the oven. I like to push down on the balls to flatten them just a little prior to baking.
Bake for 10-12 minutes until just lightly golden on the edges.
After 3 minutes, remove the cookies with a spatula and place them onto a wire rack to cool.