Chop the chocolate and pour 200 grams into one bowl and 75 grams into another.
Once the butter has cooled to a stiff but spreadable texture, add in both sugars, vanilla, and salt.
Use the paddle attachment or a hand mixer to cream the butter and sugar mixture on medium speed for about 3 minutes until it becomes a few shades lighter. Scrape the sides and bottom of the bowl.
Crack in the egg and egg yolk and mix again until smooth and fluffy.
Dump in the flour and then the baking soda and baking powder.
Use the mixer to mix on low until just combined. It's ok if some flour is still visible.
Add in the oats and 200 grams of the chocolate and mix them in just briefly.
I like big cookies so I used a quarter cup ice cream scoop. You can do any size you want, you'll simply need to adjust the baking time accordingly.
Line a small tray or baking sheet with parchment paper or a silicone baking mat and scoop the cookie dough on.
Once all the dough has been scooped, working with one cookie dough ball at a time, gently flatten the cookie so that it's still at least 1" thick.
Press the top of the cookie into the remaining 75 grams of chopped chocolate. Bring the edges up around it so that you are almost making a well of chocolate in the center of the cookie.
Place the shaped cookies back on the lined tray and pop them in the fridge for a couple hours or the freezer for at least 30 minutes. for 10 minutes or the fridge for at least 30 minutes. If you're leaving them in the fridge overnight, cover with a lid in a container or wrap with plastic.