Preheat oven to 350 degrees F. Line a baking sheet with parchment paper or a silicone mat.
Make the flax egg in a small bowl. Add the ground flaxseed and water and stir until well combined. Set aside for 5 minutes to thicken.
In the bowl of your stand up mixer (I used my KitchenAid), add the melted butter, Biscoff spread, organic brown sugar, and organic cane sugar. Use the paddle attachment and mix until well combined for about 1-2 minutes.
Add the flax egg and vanilla. Mix again until well combined. Add the flour, oats, salt, baking soda, and baking powder. Mix until combined. Add the cornflakes and chopped Biscoff cookies. Mix until combined. I like to pulse my mixer (high speed for a few seconds, then off) to mix without over mixing.
Scoop about 2 Tbsp of dough and roll it into a ball. Place on the baking sheet and slightly flatten each dough ball with your palm. Place extra chopped Biscoff cookies on top if desired.
Place the baking sheet in the freezer for 15 minutes before baking. Then bake for 10-15 minutes, depending on how big you made your cookies. Make sure you don’t over bake, the middle should still be a little soft when you pull them out of the oven. The cookies will firm up as they cool on the pan. Sprinkle flaky sea salt on top. Let them cool on the pan for at least 15 minutes before placing them on a wire rack. Enjoy!