Preheat the oven to 180 degrees Centigrade.
Add milk to flax seed powder and let it rest for 5 min.
Cream butter and sugar. Mix all-purpose flour, salt, and baking powder in a bowl and keep ready. Next, add the flaxed egg, vanilla, Nutella and mix well.
Slowly add the flour-baking powder mix, a couple of spoons at a time, mixing thoroughly to get a smooth ball of cookie dough.
Using a cookie scoop, scoop out cookie dough. Line a baking tray with parchment and arrange these balls 2 in. apart. Slightly pat it flat and bake for 8-9 for a soft chewy cookie.
Remove from the oven and cool completely. Repeat baking the next batch.
Store in an airtight container until use.