Preheat the oven to 350F (175C).
Add the butter to a saucepan and brown it on the stove over medium to low heat, continuously stirring until it turns a light golden brown color; it will continue to darken as you remove it from the heat, so don't let it get too dark! Pour this into a bowl and let it cool while you work on the dry ingredients.
Add the oats, flour, baking powder, baking soda, and salt to a bowl. Whisk to combine, then set aside.
Into the browned butter pan or in a separate bowl, add the brown sugar and the granulated sugar, the eggs, and vanilla extract; add in the brown butter if mixing in a separate bowl.
Whisk together until it looks like a caramel sauce.
Chop the chocolate into chunks if you're using a bar of chocolate or measure out your chocolate chips.
Give the raspberries a rough chop.
Add the dry ingredients into the wet ingredients, folding to combine until no dry spots remain.
Add the raspberries and white chocolate, and any other mix ins, to the cookie dough.
Fold in the chocolate and the raspberries gently.
You can chill this in the fridge to bake a little bit later if you would like, or if the mixture is very runny.
Prepare two baking sheets with parchment paper or a silicone baking mat.
Divide the cookie dough into 12 equal-sized balls. These make large, bakery-sized cookies - feel free to make smaller ones! We baked this on two baking sheets with six cookies on each, as they do spread a lot.
Add reserved raspberries (we break them apart with our hands) and chocolate chips/chunks on top of the cookies, just for extra decor. Top with demerara or pearl sugar if you would like for a bit of extra crunch.
Bake the cookies for at least 15 minutes; we bake our two baking sheets at the same time and swap them at about the ten-minute mark. If the cookies feel super soft and impossible to move off the baking sheet, give them a few more minutes at a time. They may need up to 20 minutes of baking time depending on your oven. Remember - if you made your cookies smaller, they'll need a shorter baking time.
Leave the cookies on the baking sheets for a few minutes after removing them from the oven to set up a little bit.
Remove onto a wire cooling rack; they should set up more as they cool. Enjoy!