In a large mixing bowl, whisk 230 g softened butter with 200 g white sugar and 200 g brown sugar till creamy.
Whisk in 2 eggs, 2 teaspoons vanilla extract and ½ teaspoon finely ground salt.
Dissolve 1 teaspoon baking soda in 1 teaspoon warm water and stir till it dissolves, then add to mixture blending well.
Fold in 75 g unsweetened cocoa powder and 280 g flour, then knead gently to make a dough.
Cover and refrigerate for minimum of 30 minutes.
Roll walnut size amounts, make an indentation and spoon ½ teaspoon Nutella before resealing. Flatten slightly and place on a lined baking tray, leaving approximately 3-inches between them.
Bake for 8-10 minutes till the edges are done then transfer to a cooling rack.
Notes
Refrigerating the cookie dough for at least 30 minutes (up to 3 days) prior to baking gives these results:
The moisture from the wet ingredients in the dough gets a chance to absorb properly into the dry ingredients, making the dough less sticky
The butter will take longer to melt in the oven, limiting the spread of the cookies. As the cookies crisp up on the outside, they are not able to continue spreading. This keeps them thicker.
The flavour of the cookies improve the longer the dough is kept refrigerated and they develop a wonderful rich caramel flavour.