Line two baking sheets with parchment paper.
Make the flax egg by stirring together the ground flaxseed and water. Let it sit until thickened, about 5-10 minutes.
In a large bowl, beat the butter, brown sugar, and granulated sugar until smooth and slightly lighter, about 2-3 minutes.
Mix in the flax egg, yogurt, and vanilla until combined.
In a separate bowl, whisk together the flour, cornstarch, baking soda, and fine sea salt.
Add the dry ingredients to the butter mixture and mix just until a soft dough forms.
Fold in the mixed chocolate, saving a handful for the tops.
Cover and chill the dough for at least 45 minutes.
Preheat the oven to 350°F.
Portion the dough into 70-75 g balls and place them on the prepared baking sheets with room to spread.
Press a few extra chocolate pieces onto the tops.
Bake for 11-13 minutes, or until the edges are set and lightly golden while the centers still look soft.
Sprinkle with Maldon sea salt as soon as the cookies come out of the oven.
Let the cookies cool on the baking sheet for 10 minutes, then move them to a rack to finish cooling.
