Melt the unsalted butter in a small saucepan over medium-low heat until it browns, about 5 minutes. Remove from heat and let it cool. (Tip: Watch closely to prevent burning!)
Optional: Toast the walnuts by preheating the oven to 350°F (175°C) and toasting the walnuts in a baking dish for 5-7 minutes. Let them cool.
In a medium bowl, whisk together the all-purpose flour, cinnamon, nutmeg, cardamom, salt, and baking soda. Set aside.
In a separate bowl, combine the cooled brown butter, light brown sugar, white sugar, molasses, and vanilla extract. Whisk until smooth.
Add the eggs to the wet ingredients and mix until smooth.
Mix the dry ingredients into the wet ingredients, then add the oats, cranberries, and toasted walnuts. Stop mixing once combined.
Chill the cookie dough in the fridge for 45 minutes to firm up. (This helps prevent spreading during baking!)
Preheat the oven to 350°F (175°C) and line 2 baking sheets with parchment paper.
Using a 3-tablespoon cookie scoop, place the dough on the baking sheets with 3-4 inches between each cookie. Bake 6 cookies at a time for 11-13 minutes, until the edges are set but the centers are slightly underbaked.
Let the cookies cool on the baking sheet for 5 minutes before transferring them to a cooling rack.
Notes
For extra flavor, you can add a pinch of sea salt on top of each cookie before baking.