Melt the butter in a light-colored saucepan over medium heat, stirring constantly. Continue cooking until the butter turns golden brown and smells nutty, about 5-7 minutes. Watch carefully so it doesn't burn!
Pour the brown butter into a heatproof bowl and let it cool to room temperature, about 30 minutes. You can speed this up by placing the bowl in the refrigerator for 15 minutes.
Once cooled, add both sugars to the brown butter and beat with an electric mixer until well combined, about 2 minutes.
Beat in the eggs one at a time, then add the vanilla extract and mix until smooth.
In a separate bowl, whisk together the flour, baking soda, baking powder, salt, and cinnamon.
Add the dry ingredients to the wet ingredients and mix on low speed until just combined. The dough will be soft.
Cover the dough and refrigerate for at least 30 minutes (or up to 3 days) to firm up.
When ready to bake, preheat your oven to 350°F and line baking sheets with parchment paper.
Scoop rounded tablespoons of dough and roll into balls. Place them 2 inches apart on the baking sheets. Sprinkle with flaky sea salt if desired.
Bake for 10-12 minutes until the edges are golden and the centers still look slightly soft.
Let cookies cool on the baking sheet for 5 minutes before transferring to a wire rack. Enjoy warm or store in an airtight container!