Cream the butter, dark brown sugar, and granulated sugar in a large bowl until light and fluffy (2-3 minutes).
Add the eggs one at a time and mix well. Then stir in the vanilla extract.
In a separate bowl, whisk together oats, flour, salt, baking soda, and baking powder.
Gradually mix the dry ingredients into the wet ingredients until just combined.
Fold in the chocolate chips, chopped milk chocolate, and nuts if using.
Chill the dough for at least 30 minutes in the refrigerator (optional).
Preheat the oven to 350°F (175°C).
Scoop mounds of dough onto prepared baking sheets, spacing them 2 inches apart. Bake for 10-12 minutes.
Cool on the baking sheet for a few minutes before transferring to a cooling rack.
