Can I use a different filling besides brownie batter?

Yes you can use any kind of dough or spread you wish! Biscoff spread, oatmeal butter, peanut butter, Nutella, etc!

How do I store these cookies?

Keep cookies stored in an airtight container. Stored well, they will remain fresh for 3-4 days at room temperature or for 6-7 days if stored in the fridge.

Can I use a sugar substitute (like monk fruit or Truvia) instead of brown sugar for these cookies?

I’ve tested this recipe with light brown sugar, golden classic monk fruit sweetener and Truvia brown sugar. The real brown sugar does some science-type stuff in this recipe and really plays an important part in creating the most soft and chewy cookie. I’ve just found the cookies to be a little harder (and not as chewy) when using the substitute over the real stuff. So I would really recommend the real stuff in this recipe! But if you need to bring the sugar down for health reasons, you can use a sugar substitute instead (if you do this, definitely warm the cookie up before eating it to help make it softer!)