Brown the butter in a light-colored saucepan over medium heat, stirring frequently until golden brown specks appear and the butter smells nutty. Immediately transfer to a heatproof bowl and cool for about 10–15 minutes.
In a large mixing bowl, whisk together the browned butter, dark brown sugar, and granulated sugar until smooth and glossy.
Add the eggs one at a time, mixing well after each addition. Stir in the vanilla.
In a separate bowl, whisk together the flour, baking soda, baking powder, sea salt, cinnamon, nutmeg, cloves, and cardamom (if using).
Gradually fold the dry ingredients into the wet ingredients until just combined. Avoid overmixing.
Fold in the rolled oats and any optional mix-ins until evenly distributed.
Cover the dough and refrigerate for at least 30 minutes.
Preheat your oven to 350°F (175°C) and line baking sheets with parchment paper.
Using a large cookie scoop (about 2½ tablespoons), portion the dough onto the prepared baking sheets, leaving about 3 inches between each cookie. Lightly sprinkle each dough ball with flaky sea salt.
Bake for 10–12 minutes, or until the edges are lightly golden while the centers still look slightly underbaked.
Allow the cookies to rest on the hot baking sheet for 3–5 minutes before transferring them to a wire rack to cool completely.