Combine 1 ¼ tablespoon of boiling water with chia seeds to make a chia egg. Stir very well, then set aside for later. In the meantime, melt the butter in a large mixing bowl and add in the brown and white sugar. Mix the butter and sugar mixture until it thickens slightly and sugar has partly dissolved.
Add the vanilla and chia egg to the butter mixture and stir.
In a separate bowl mix together the flour, bicarb, corn starch, salt, and matcha powder.
Gradually add the flour mixture to your butter mixture and stir with a spatula until just combined. Chop your hazelnuts and white chocolate and fold into the matcha cookie dough.
Cover your bowl with clingfilm and let the dough rest in the fridge for at least an hour.
After an hour, scoop the mixture into 8 equal balls and place on a baking tray.
Bake for 10-12 minutes at 180°C/356°F (fan oven) and let cool on baking tray. If not using a fan oven, bake at 200°C/392°F.