Use a good, pure maple syrup. The better the syrup the more the maple flavor shines in these cookies.
Melt the butter in a heavy-bottomed skillet set over medium heat, whisking constantly, then cook until lightly browned bits form in the bottom of the skillet and the butter is light tan in color with a nutty aroma, about 8 minutes. Remove from heat; butter will continue to brown even after removed from heat. Transfer butter to a medium bowl and place in the refrigerator. Chill for at least 1 hour.
Adjust oven racks to upper and lower third of the oven and preheat oven to 350°. Line two baking sheet pans with parchment paper.
In a medium bowl, whisk together the oats, flour, baking powder, salt and cinnamon. Set aside.
To the bowl of a stand mixer fitted with the paddle attachment, add the browned butter, dark brown sugar and sugar (Alternately, use a large bowl and a hand-held electric-mixer.) Beat on high until the mixture is fluffy, about 3 minutes. Scrape down sides of the bowl with a spatula. Beat in the eggs, vanilla and maple syrup until just combined. With mixer on low, add the oat mixture in 2 batches, beating just until combined.
Drop 2-inch balls of dough onto prepared baking sheets using a tablespoon or small ice-cream scoop. Flatten slightly using the bottom of a glass. Bake, rotating and baking sheets midway through baking, until cookies are uniformly light brown, 12 to 14 minutes. Let cookies cool on baking sheets until set, about 2 minutes, then transfer to a wire rack and let cool completely. Serve.The dough can be refrigerated for up to three days or frozen for up to two months.
The dough can be refrigerated for up to three days or frozen for up to two months.