Preheat the oven to 375 degrees F. Line 2 baking sheets with parchment paper.
In the bowl of a stand mixer fitted with the paddle attachment (or in a large bowl with a hand mixer), cream the butter and brown sugar until light and fluffy, 3 to 4 minutes. With the mixer running on low speed, add the maple syrup in a slow, steady stream and mix to combine.
Scrape the bowl well, then add the egg and mix on medium speed until the mixture is well combined. Beat in the vanilla.
Add the flour, baking powder and fine salt to the mixer and pulse until it starts to combine, then scrape the bowl well and mix on low speed until uniformly combined. Add the pecans and mix until evenly incorporated.
Scoop the dough into 2 tablespoon-size rounds on the prepared baking sheets, staggering the rows and leaving at least 1 1/2 inches between each cookie to allow room to spread. Sprinkle flaky sea salt on the surface of each cookie. Transfer to the oven and bake until the cookies are lightly golden around the edges, 15 to 18 minutes. Transfer to a cooling rack to cool before serving.