Whisk together both flours, baking soda, baking powder, salt, and nutmeg in a large bowl. Set aside.
Fit your stand mixer with the paddle attachment and then cream together the butter and sugar for about 5 minutes, or until light and airy. Add the vanilla and give it a good stir.
Crack in the eggs and egg yolk, one at a time, and mix for 30 seconds in between each addition.
Add the dry ingredients in two parts, stirring each addition just until the flour disappears.
Fold in the hazelnuts and apricots on a low speed until well dispersed.
Cover the bowl with tin foil and refrigerate for a minimum of 3 hours. For maximum flavour, it’s best to refrigerate overnight.
When you’re ready to bake the cookies, remove the dough from the fridge, preheat the oven to 180°C (350°F) and line two baking trays with silicone mats or non-stick parchment paper. Allow the dough to come to room temperature while the oven is preheating (about 15 minutes) before baking.
Roll the dough into roughly 3-tablespoon sized balls. They will be quite large – don’t panic. Drop each dough ball onto the tray, making sure to leave ample room between each cookie as they spread in oven. I suggest no more than 8 dough balls per tray.
Bake the cookies for 11-13 minutes. They should be brown around the edges and on the bottom and have puffed up at the centre.
Remove the tray from the oven and allow the cookies to sink and cool for 10 minutes. Move the cookies to a wire rack to finish cooling.